Run the tests with cgo off and without the race detector (closes #538)

sneak ruled the race detector out
(#538 (comment)), and on
Linux it was the only reason the tests needed cgo. The test phase and the
browser test binary now build with CGO_ENABLED=0 and without -race, so no
test binary compiles mattn's C SQLite. The test phase keeps the builder
stage's Debian Go image. Its comments, the README and the code comments that
said the suite runs under -race now say what is true, and the README notes
the exception to the vendored REPO_POLICIES.md.

Model: opus-5-5

internal/archive/evict_test.go

@@ -303,7 +303,7 @@ func (r *racingWrites) wait() (bool, error) {
 // contending for the writer's mutex when the eviction takes it.
 // The write that wins the mutex after the eviction must abandon
 // its work rather than reopen the archive, leaving the writer
-// permanently handle-free. Run under -race.
+// permanently handle-free.
 func TestEvictWebhook_RacingWriteDoesNotReopenHandle(
 	t *testing.T,
 ) {