Switch coverage CI targets to EngFlow (#39269)

Commit Message:

Currently Envoy CI uses different RBE backends for different CI targets.
EngFlow is one of available backends and we want to migrate most if not
all targets to EngFlow.

Other than just making the overall CI setup simpler, EngFlow appear to
offer more powerful build grid machines at the moment, plus some
additional nice features like automatically scaling memory when build
tasks don't fit into available memory.

One specific reason why I'd like to migrate coverage targets to EngFlow
is because I want to switch them to static linking to workaround a bug
in Clang/LLVM source-based coverage (see llvm/llvm-project#32849).

With currently used Google RBE backend we are having issues with fuzzing
coverage tests, as fuzzing tests include a lot of extensions and
together with coverage instrumentation it pushes linker memory footrpint
way to high and causing OOMs.

Our approach to solving this particular problem is two-fold:

1. I want to migrate to EngFlow that can offer bigger machines (and it
aligns with the general direction for Envoy CI to migrate to EngFlow.
2. I want to optimize our fuzzing targets a little bit by cutting out
some unnecessary bits and reducing the number of libraries that linker
need to link together (on top of reducing the amount of time it takes to
build things and similar benefits).

This particular PR takes care of the first part.

Additional Description:

Some relevant discussions can be found in
#39030 which prompted me to work
on this in the first place. And I will use
#39248 as a tracking bug for
the coverage changes.

Risk Level: medium (it could break CI and cause disruption, but rolling
it back should be easy)
Testing: I applied the same change in the envoy-ci-staging repo and
created a test PR to see if EngFlow is used for coverage. The setup
between envoy-ci-staging and envoy repo is not exactly the same, so I
could have missed something.

---------

Signed-off-by: Mikhail Krinkin <[email protected]>

Author: krinkinmu

.github/workflows/_check_coverage.yml

@@ -38,7 +38,7 @@ jobs:
     uses: ./.github/workflows/_run.yml
     name: ${{ matrix.name ||matrix.target }}
     with:
-      # bazel-extra: '--config=remote-envoy-engflow'
+      bazel-extra: '--config=remote-envoy-engflow'
       cache-build-image: ${{ fromJSON(inputs.request).request.build-image.default }}
       concurrency-suffix: -${{ matrix.target }}
       diskspace-hack: ${{ matrix.diskspace-hack && true || false }}
@@ -50,7 +50,6 @@ jobs:
         lower than limit
       gcs-cache-bucket: ${{ inputs.gcs-cache-bucket }}
       rbe: true
-      rbe-google: true
       request: ${{ inputs.request }}
       runs-on: ${{ fromJSON(inputs.request).config.ci.agent-ubuntu }}
       steps-post: |