`pkgcache` used for building leads to failures

[arnetheduck]

It seems that pkgcache is used for some build steps - this means that whatever the build process produces stays in the pkgcache checkout and later disrupts commands such as git submodule update:

  Warning:  Error reading tag v0.0.7: for package libbacktrace. This may not be relevant as it could be an old version of the package. 
        ...  Execution of 'git -C /home/arnetheduck/.nimble/pkgcache/githubcom_statusimnimlibbacktrace_versions submodule update --init --recursive --depth 1' failed with an exit code 1.
        ... Details: error: Your local changes to the following files would be overwritten by checkout:
        ... 	configure
        ... Please commit your changes or stash them before you switch branches.
        ... Aborting
        ... fatal: Unable to checkout '95e8e96c86349815b86001171d090a76bd0e1ddb' in submodule path 'vendor/libbacktrace-upstream'

The pkgcache should not have a working tree at all - ie its role is to cache the git data so that we quickly can check out various branches and tags but ideally, it would not have a worktree at all (except maybe temporarily, to parse the nimble file) - see --bare/--no-checkout of git clone.