Use stub-resolv.conf for /etc/resolv.conf

build-inside-vm.sh:
Change the source of `/etc/resolv.conf` to point to
`/run/systemd/resolve/stub-resolv.conf`, as it provides support for
per-interface resolving [1].

Fixes #123

[1] https://www.freedesktop.org/software/systemd/man/systemd-resolved.service.html#/etc/resolv.conf
6 jobs for issues/123 in 28 minutes and 34 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Lint
passed shellcheck #10732

00:00:13

passed shfmt #10733

00:00:13

 
  Build
passed build #10737

00:23:51

failed build #10734

00:00:53

 
  Test
passed test-cloudimg-qemu #10736

00:04:28

passed test-vagrant-boxes-format #10735

00:00:43