Fix conflict on heredoc temp files

[kmichel-aiven]

What type of PR is this?

/kind bug

What this PR does / why we need it:

If two image builds start concurrently and they both use an heredoc with the same EOF marker, then they will use the same filename and overwrite each other.

It's possible for concurrent builds to either use the wrong heredoc content or to fail because the file of one build is deleted by the other, leading to this error message:

STEP 13/21: COPY <<EOF /file.conf

Error: building at STEP "COPY <<EOF /file.conf": reading "/var/tmp/EOF": copier: get: globs [/EOF] matched nothing (0 filtered out): no such file or directory

How to verify it

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

[kmichel-aiven]

I would like to add tests but I'm not familiar with the project, what would be the best place to add these ?

(I've tried adding to bud.bats but run_buildah doesn't seem ready to test concurrent operations.)

[github-actions]

A friendly reminder that this PR had no activity for 30 days.

[TomSweeneyRedHat]

@edsantiago thoughts on tests ?

[edsantiago]

I see no reason for tests. However, @kmichel-aiven , could you please clean up your git commit, and rebase?

[openshift-ci]

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This pull-request has been approved by: kmichel-aiven
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