[ci] fix valgrind workflow

[jameslamb]

Replaces #6815

The valgrind CI job is failing like this:

/__w/_temp/a09e15f1-9453-4607-9d54-66e1c2404ea4.sh: 6: comment+=https://github.com/microsoft/LightGBM/actions/runs/13159252384: not found

(build link)

This attempts to fix that.

Notes for Reviewers

As described in #6743, += is a non-POSIX behavior.
My guess is that using it in run: in GitHub Actions steps is problematic if the shell is not one that recognizes +=.

It should be the case that GitHub Actions is using bash in these jobs, which shouldn't be a problem. Per https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#defaultsrunshell

shell unspecified

[bash -e {0} is] the default shell on non-Windows platforms. Note that this runs a different command to when bash is specified explicitly. If bash is not found in the path, this is treated as sh.

I'm not sure why (or even if) bash is not being found. The container image for the valgrind job definitely does have it.

docker run \
  --rm \
  -it wch1/r-debug \
  bash --version

# GNU bash, version 5.1.16(1)-release (x86_64-pc-linux-gnu)

This PR attempts to fix that job in 2 ways:

• removing uses of += in Github Actions workflow steps
• explicitly setting shell: bash in those steps

How to Test This

I think that we'll have to merge this to test it, because the comment-triggered workflows pull configuration from master: #6815 (comment)

[StrikerRUS]

Ah, thanks a lot for this PR!

Indeed, I forgot about that too

I forgot that for these comment-triggered workflows, the GitHub Actions config is sourced from master. We'll have to merge these changes to test them.
#6815 (comment)

and my original check in #6758 (comment) made no sense. I'm really so sorry about that!

[jameslamb]

and my original check in #6758 (comment) made no sense. I'm really so sorry about that!

It's ok! I reviewed #6758 and also forgot about this 😅

[jameslamb]

This worked!

#6802 (comment)