Only use `DIST_TRY_BUILD` for try jobs that were not selected explicitly
Some CI jobs (x64 Linux, ARM64 Linux and x64 MSVC) use the `opt-dist` tool to build an optimized toolchain using PGO and BOLT. When performing a default try build for x64 Linux, in most cases we want to run perf. on that artifact. To reduce the latency of this common use-case, `opt-dist` skips building several components not needed for perf., and it also skips running post-optimization tests, when it detects that the job is executed as a try job (not a merge/auto job).
This is useful, but it also means that if you *want* to run the tests, you had to go to `jobs.yml` and manually comment this environment variable, create a WIP commit, do a try build, and then remove the WIP commit, which is annoying (in the similar way that modifying what gets run in try builds was annoying before we had the `try-job` annotations).
I thought that we could introduce some additional PR description marker like `try-job-run-tests`, but it's hard to discover that such things exist.
Instead, I think that there's a much simpler heuristic for determining whether `DIST_TRY_BUILD` should be used (that I implemented in this PR):
- If you do just ``@bors` try`, without any custom try jobs selected, `DIST_TRY_BUILD` will be activated, to finish the build as fast as possible.
- If you specify any custom try jobs, you are most likely doing experiments and you want to see if tests pass and everything builds as it should. The `DIST_TRY_BUILD` variable will thus *not* be set in this case.
In this way, if you want to run dist tests, you can just add the `try-job: dist-x86_64-linux` line to the PR description, and you don't need to create any WIP commits.
r? `@marcoieni`
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