docs: update build_stage examples

[tldahlgren]

See #12072

Update more build_stage examples due to the potential impact of spack clean should the old example paths be used.

[mwkrentel]

@tldahlgren @scheibelp @tgamblin
IMO this patch and #12492 misunderstand the problem here.

The problem is not a better error message or better documentation.
The problem is that you reduced the options for build_stage to
ONE option and that option DOES NOT WORK.

build_stage:
- $tempdir/spack-stage

Here, you expect $tempdir to have permissins like 1777 and so you
can create new directories there. But /tmp/spack-stage does not
have mode 1777. It's owned by another user. This has become a race
condition where it works for the first person to run spack on this
machine and then fails for the other N-1.

You need to restructure the temp dirs so that each user has their own
copy of spack-stage that is writeable for that user.

[tldahlgren]

@tldahlgren @scheibelp @tgamblin
IMO this patch and #12492 misunderstand the problem here.

The problem is not a better error message or better documentation.
The problem is that you reduced the options for build_stage to
ONE option and that option DOES NOT WORK.

build_stage:
- $tempdir/spack-stage

The idea was there was a single "default" value which the user could override. (I was under the impression that was often the case).

Here, you expect $tempdir to have permissins like 1777 and so you
can create new directories there. But /tmp/spack-stage does not
have mode 1777. It's owned by another user. This has become a race
condition where it works for the first person to run spack on this
machine and then fails for the other N-1.

You need to restructure the temp dirs so that each user has their own
copy of spack-stage that is writeable for that user.

There are systems that automatically add $user to $tempdir. (That is clearly not the case in your environment but they do exist.) In order to avoid the potential for multiple $user nodes in the path, staging is supposed to be appending $user to the first accessible build_stage path when establishing the stage root directory. Unfortunately, I now see I missed the case where the path exists but the user cannot access it.

[tldahlgren]

Closing since, after much internal discussion, the build_stage path is going to be re-worked.