imagebuildah.StageExecutor: clean up volumes/volumeCache

[nalind]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Clean up the distinctions between the volumes slice and the volumeCache and volumeCacheInfo maps so that --compat-volumes will work correctly when we're building with multiple layers.

How to verify it

Expanded integration tests!

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

The `buildah build` `--compat-excludes` flag will now how the intended effect when used in combination with `--layers`.

[rhatdan]

LGTM
@flouthoc PTAL

[rhatdan]

@giuseppe @mheon PTAL

[giuseppe]

/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: giuseppe, nalind

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[nalind]

/cherry-pick release-1.37

[openshift-cherrypick-robot]

@nalind: #5729 failed to apply on top of branch "release-1.37":

Applying: imagebuildah.StageExecutor: clean up volumes/volumeCache
Using index info to reconstruct a base tree...
M	cmd/buildah/build.go
M	imagebuildah/build.go
M	imagebuildah/executor.go
M	imagebuildah/stage_executor.go
M	tests/bud.bats
Falling back to patching base and 3-way merge...
Auto-merging tests/bud.bats
Auto-merging imagebuildah/stage_executor.go
CONFLICT (content): Merge conflict in imagebuildah/stage_executor.go
Auto-merging imagebuildah/executor.go
Auto-merging imagebuildah/build.go
Auto-merging cmd/buildah/build.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 imagebuildah.StageExecutor: clean up volumes/volumeCache
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick release-1.37

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