[build] Disable WebGPU for Windows non-release builds

The Windows build is currently much slower than the macOS/Linux builds on CI,
especially on mostly cached builds.
Based on profiling on #2981, this is caused by the repo mapping stage for dawn
taking several minutes of Windows. Disable the experimental WebGPU feature on
Windows to better meet compile time constraints. Release builds remain
unaffected.

.bazelrc

@@ -27,8 +27,12 @@ build:windows --remote_download_all
 # consider moving more flags out to separate files.
 import %workspace%/build/ci.bazelrc
 
-# Enable webgpu
-build --//src/workerd/io:enable_experimental_webgpu=True
+# Enable webgpu. Merely fetching the required Dawn library and computing the repo mapping based on
+# it slows down the Windows build significantly, so only enable this on Unix and for Windows release
+# builds. This is mostly motivated by compile time constraints, we hope to lift this limitation
+# again if this Windows performance bottleneck gets fixed within Bazel.
+build:unix --//src/workerd/io:enable_experimental_webgpu=True
+build:release_windows --//src/workerd/io:enable_experimental_webgpu=True
 
 # Avoid generating duplicate runfile trees. This will become the default in a future bazel version.
 build --nolegacy_external_runfiles