Currently, the Roachtest Stress TeamCity job uses `make` (via the
`mkrelease.sh` script) for building the required binaries. Recently, the
pipeline started hanging while building the binary. Rather than
investigating and fixing the existing `make`-based pipeline, convert the
job to using Bazel for building the binaries.

Retain the existing entrypoint to the job, converting it to use [the
existing pattern][1] of calling `run_bazel` with the required
environment variables, invoking an `_impl.sh` script. The existing logic
is moved into the new `_impl.sh` script, with some minor additions to
source some additional scripts.

Additionally, the TeamCity job itself was updated to set pass in the
`TARGET_CLOUD` environment variable (set to `gce`), and remove the need
to build within a container (instead deferring to Bazel to manage the
build environment). The diff for the TeamCity pipeline is as follows
(for posterity):

```diff
18c18
<
---
>       <param name="env.TARGET_CLOUD" value="gce" />
24c24
<           <param name="plugin.docker.imageId" value="%builder.dockerImage%" />
---
>
28,29c28,29
< export BUILD_TAG="$(git describe --abbrev=0 --tags --match=v[0-9]*)"
< ./build/teamcity-roachtest-stress.sh]]></param>
---
> build_tag="$(git describe --abbrev=0 --tags --match=v[0-9]*)"
> CLOUD=${TARGET_CLOUD} BUILD_TAG="${build_tag}" ./build/teamcity-roachtest-stress.sh]]></param>
```

Release note: None.

[1]: https://github.com/cockroachdb/cockroach/blob/12198a51408e7333cd4f96b221e6734239479765/build/teamcity/cockroach/nightlies/roachtest_nightly_gce.sh#L10-L11