use clang for coverage in CI (#121)

it is a bit faster than GCC

.bazelrc

@@ -30,3 +30,8 @@ build --workspace_status_command=tools/bazel_workspace_status.sh
 # when triggering builds using different shells on the same machine.
 # https://stackoverflow.com/questions/74881594/bazel-builds-from-scratch-ignoring-cache
 build --incompatible_strict_action_env
+
+coverage --combined_report=lcov
+coverage --experimental_generate_llvm_lcov
+# Needed because our tests are in a different package than the code they test.
+coverage --instrumentation_filter='//.*'

.github/workflows/ci.bazelrc

@@ -9,15 +9,3 @@ build --remote_download_minimal
 
 test --test_output=all
 test --test_verbose_timeout_warnings
-
-coverage --combined_report=lcov
-coverage --strategy=CoverageReport=local
-# At least some of this is needed for the coverage tool to work.
-coverage --experimental_split_coverage_postprocessing
-coverage --experimental_fetch_all_coverage_outputs
-coverage --remote_download_outputs=all
-coverage --experimental_remote_download_regex=.*/((testlogs/.*/_coverage/.*)|coverage.dat$|_coverage/_coverage_report.dat$)
-# Not sure why it doesn't work with clang. Should be possible.
-coverage --extra_toolchains=//toolchain:gcc
-# Needed because our tests are in a different package than the code they test.
-coverage --instrumentation_filter='//.*'