Avoid generate duplicated tasks (#2093)

Previously, bazel version defined in the matrix was still expanded via
the "old_bazel" attributes, leading to duplicated tasks:

-
https://buildkite.com/bazel/bcr-bazel-compatibility-test/builds/22#0192d96c-70d2-4057-a659-c9944b421a0b
-
https://buildkite.com/bazel/bcr-bazel-compatibility-test/builds/22#0192d96c-70dc-458b-bd72-637d99898ac5

This PR flattens bazel versions in the matrix to a single string to
avoid that while still keeping a nice warning message like
```
Bazel version overridden from 6.x, 7.x to 8.0.0rc2
```
1 file changed