refactor: pass mode and devtool as environment variables

[acozzette]

With use_execroot_entry_point = False, we want Webpack config files to be built for the exec platform. However, this poses a problem for mode and devtool, which are derived from the compilation mode and are currently baked into the base config file. If we build that config file for the exec platform, then it cannot be customized based on the target platform compilation mode.

This commit solves that problem by passing this information via WEBPACK_MODE and WEBPACK_DEVTOOL environment variables. This way we can get the information we need about the compilation mode without having to hard-code it in the generated base config.

Importantly, user-provided configs can still override mode and devtool as expected, and there is even an existing test for this in webpack/tests/devtool-override.

---

Changes are visible to end-users: no, unless you go out of your way to look at these environment variables

Test plan

• Covered by existing test cases

[aspect-workflows]

Bazel 7 (Test)

15 test targets passed

Targets

//.github/workflows:bazel7.update_test [k8-fastbuild]                        87ms
//docs:update_test [k8-fastbuild]                                            26ms
//tools:preset.update_test [k8-fastbuild]                                    79ms
//webpack/tests/create_configs:tests_0_test [k8-fastbuild]                   69ms
//webpack/tests/create_configs:tests_1_test [k8-fastbuild]                   69ms
//webpack/tests/create_configs:tests_2_test [k8-fastbuild]                   67ms
//webpack/tests/create_configs:tests_3_test [k8-fastbuild]                   71ms
//webpack/tests/devserver-configure:test_dbg [k8-fastbuild]                  61ms
//webpack/tests/devserver-configure:test_fastbuild [k8-fastbuild]            47ms
//webpack/tests/devserver-configure:test_opt [k8-fastbuild]                  62ms
//webpack/tests/devtool-override:write_bundle_test [k8-fastbuild]            66ms
//webpack/tests/simple:bundle_no_execroot_entry_point_matches [k8-fastbuild] 38ms
//webpack/tests/simple:entries_match [k8-fastbuild]                          71ms
//webpack/tests/simple:write_bundle_test [k8-fastbuild]                      46ms
//webpack/tests/worker:write_bundle_test [k8-fastbuild]                      67ms

---

Bazel 8 (Test)

All tests were cache hits

12 tests (100.0%) were fully cached saving 864ms.

---

Bazel 9 (Test)

All tests were cache hits

12 tests (100.0%) were fully cached saving 1s.

---

Bazel 7 (Test)

e2e/loaders

1 test target passed

Targets

//:test [k8-fastbuild]                                                       21ms

---

Bazel 8 (Test)

e2e/loaders

All tests were cache hits

1 test (100.0%) was fully cached saving 36ms.

---

Bazel 9 (Test)

e2e/loaders

All tests were cache hits

1 test (100.0%) was fully cached saving 31ms.

---

Bazel 7 (Test)

e2e/loaders_jslib

1 test target passed

Targets

//:test [k8-fastbuild]                                                       23ms

---

Bazel 8 (Test)

e2e/loaders_jslib

All tests were cache hits

1 test (100.0%) was fully cached saving 62ms.

---

Bazel 9 (Test)

e2e/loaders_jslib

All tests were cache hits

1 test (100.0%) was fully cached saving 32ms.

---

Bazel 7 (Test)

e2e/smoke

2 test targets passed

Targets

//src:bundles_0_test [k8-fastbuild]                                          24ms
//src:bundles_1_test [k8-fastbuild]                                          22ms

---

Bazel 8 (Test)

e2e/smoke

All tests were cache hits

2 tests (100.0%) were fully cached saving 46ms.

---

Bazel 9 (Test)

e2e/smoke

All tests were cache hits

2 tests (100.0%) were fully cached saving 124ms.

---

Bazel 7 (Test)

e2e/worker

2 test targets passed

Targets

//:bundles_match [k8-fastbuild]                                              35ms
//:write_bundle_test [k8-fastbuild]                                          34ms

---

Bazel 8 (Test)

e2e/worker

All tests were cache hits

2 tests (100.0%) were fully cached saving 141ms.

---

Bazel 9 (Test)

e2e/worker

All tests were cache hits

2 tests (100.0%) were fully cached saving 67ms.

---

Buildifier      Gazelle