Fix shellcheck complains due to new version

Shellcheck v0.7.2[1] added more checks resulting in the following
complains in our case:
In build-host.sh line 12:
  readonly TMPDIR="$(mktemp --dry-run --directory --tmpdir="${PWD}/tmp")"
           ^----^ SC2155: Declare and assign separately to avoid masking return values.
In build-inside-vm.sh line 16:
  readonly TMPDIR="$(mktemp --dry-run --directory --tmpdir="${PWD}/tmp")"
           ^----^ SC2155: Declare and assign separately to avoid masking return values.
In build-inside-vm.sh line 19:
    chown "${SUDO_UID}:${SUDO_GID}" "${OUTPUT}" "${TMPDIR}"
                       ^---------^ SC2153: Possible misspelling: SUDO_GID may not be assigned, but SUDO_UID is.

[1] https://github.com/koalaman/shellcheck/blob/331e89be990547b6e21ad1b6e56065bcda1ba053/CHANGELOG.md#v072---2021-04-19
6 jobs for fix-shellcheck in 31 minutes and 28 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Lint
passed shellcheck #21657

00:00:18

passed shfmt #21658

00:00:11

 
  Build
passed build #21659
fast-single-thread

00:24:37

 
  Test
passed test-basic-qemu #21661
fast-single-thread

00:04:44

passed test-cloudimg-qemu #21662
fast-single-thread

00:06:30

passed test-vagrant-boxes-format #21660

00:00:49