pkgctl build: add option to test-install the created package

Currently there is no option to have the package test-installed to the local system.

🚧 Current workflow:

1. pkgctl build --testing
2. sudo pacman -U pkgname.pkg.tar.zst
3. verify that the build package works as expected
4. pkgctl release ...

🚀 New workflow:

1. pkgctl build --local-install --testing
2. verify that the build package works as expected
3. pkgctl release ...

Sadly --install is already taken to install packges into the build chroot 🤔