Fix coverage for C++ in XLA. (#4984)
The root cause is very contrived. `bazel coverage` works well, and
calculates the coverage. The output `lcov` data is written and a
deterministic symlink is provided for it under `bazel-out`.
On the second run, the file disappears. The reason is that we use
a remote cache, and that we don't download all artifacts from the build,
just the toplevel artifacts. From bazel 7.0.0, coverage symlinks will
be included, but for now we have to specify that we want to download
all outputs; we only want to do this when doing coverage.
We cannot specify this in the `.bazelrc` file because the coverage
command inherits the flags from the remote build command; one of the
flags there is setting the minimal output for fetching the toplevel
objects, but it actually sets three separate flags, and we can only
override the one here, so we end up in an bad state: still downloading
the minimal output that does not yet support symlinks. Overriding the
flag on the CLI works because it gets the highest priority and overrides
the same flag from the remote build config.
This is not easy to test in the CI because on the uncached run it always
works, but on the second PR where the code coverage hash has not
changed, which is often the case, it resolves to the cached symlink that
is not fetched and fails subsequently.
