pmb.helpers.run: only pass stdin where useful
Don't pass stdin to commands that aren't supposed to be used
interactively (output: log, background, pipe).
This fixes an inconsistency between building packages in CI on gitlab
and building them via bpo on sourcehut or locally. In gitlab, apparently
there is no stdin for the entire build job and so unanswered kernel
config prompts will just use the default. In local builds and on
sourcehut stdin is available and so it just hangs at the prompt until
pmbootstrap kills the build job due to no output being written.
I considered adding an additional check to pmaports to ensure that there
are no unanswered kernel config prompts just in case users run abuild
manually on the kernel APKBUILD with stdin available. But I think
forcing the users to answer all the prompts even if it's not really
needed just creates additional work / makes the workflow worse without
real benefit.