go/tools/bazel_testing: provide a way to configure C toolchain in tests

[jayconrod]

go_bazel_tests that require cgo have been failing on Windows since the
test environment hasn't been passing additional flags needed to
configure the C toolchain on Windows.

This CL adds GO_BAZEL_TEST_BAZELFLAGS, an environment variable to be
set with --test_env, which provides a list of additional flags to
Bazel.

Fixes #2507

[meteorcloudy]

Looks like this change broke tests on Windows:
https://buildkite.com/bazel/rules-go-golang/builds/2129#eed1c52d-20e1-4990-ac86-0f0f8138e41b

Executing tests from //tests/core/race:race_test
-----------------------------------------------------------------------------
--- FAIL: Test (47.83s)
    --- FAIL: Test/cover_race (15.36s)
        race_test.go:264: running: bazel coverage //:coverrace_test
        race_test.go:272: unexpected build failure: exit status 1
            stderr:
            Loading: 
            Loading: 0 packages loaded
            INFO: Using default value for --instrumentation_filter: "//".
            INFO: Override the above default with --instrumentation_filter
            INFO: Build options --collect_code_coverage, --instrumentation_filter, and --test_timeout have changed, discarding analysis cache.
            Analyzing: target //:coverrace_test (0 packages loaded, 0 targets configured)
            INFO: Analyzed target //:coverrace_test (0 packages loaded, 7044 targets configured).
            INFO: Found 1 test target...
            [0 / 3] [Prepa] BazelWorkspaceStatusAction stable-status.txt
            [9 / 15] [Sched] GoStdlib external/io_bazel_rules_go/stdlib_/pkg
            [9 / 15] GoStdlib external/io_bazel_rules_go/stdlib_/pkg; 1s local
            [9 / 15] GoStdlib external/io_bazel_rules_go/stdlib_/pkg; 10s local
            [20 / 21] [Prepa] Testing //:coverrace_test
            FAIL: //:coverrace_test (see C:/b/4xosdfd5/execroot/__main__/bazel-out/x64_windows-fastbuild-ST-e550172346ce7259f92416ab36072baf4df0deb3e748a2c931fafa98361b25ba/testlogs/coverrace_test/test.log)
            ERROR: C:/b/gmkpsojl/bazel_testing/bazel_go_test/main/BUILD.bazel:59:8: output 'coverrace_test/coverage.dat' was not created
            ERROR: C:/b/gmkpsojl/bazel_testing/bazel_go_test/main/BUILD.bazel:59:8: not all outputs were created or valid
            Target //:coverrace_test up-to-date:
              bazel-out/x64_windows-fastbuild-ST-e550172346ce7259f92416ab36072baf4df0deb3e748a2c931fafa98361b25ba/bin/coverrace_test_/coverrace_test.exe
            INFO: Elapsed time: 15.316s, Critical Path: 14.84s
            INFO: 3 processes: 3 local.
            FAILED: Build did NOT complete successfully
            FAILED: Build did NOT complete successfully

[jayconrod]

@meteorcloudy #2519 skips the test.