shellcheck fails on .gitlab/ci/build-host.sh & .gitlab/ci/build-inside-vm.sh

The shellcheck job started to fail on 2020-04-21 when shellcheck 0.7.2-1 landed in the repos. shellcheck v0.7.2 added more checks which means our scripts need fixing.

Job #20399 failed for 1a971096:

In .gitlab/ci/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 .gitlab/ci/build-inside-vm.sh line 9:
readonly tmpdir="$(mktemp --dry-run --directory --tmpdir="${orig_pwd}/tmp")"
         ^----^ SC2155: Declare and assign separately to avoid masking return values.


In .gitlab/ci/build-inside-vm.sh line 25:
    chown "${SUDO_UID}:${SUDO_GID}" "${1}"{,.b2,.sha{256,512}}
                       ^---------^ SC2153: Possible misspelling: SUDO_GID may not be assigned, but SUDO_UID is.

For more information:
  https://www.shellcheck.net/wiki/SC2153 -- Possible misspelling: SUDO_GID ma...
  https://www.shellcheck.net/wiki/SC2155 -- Declare and assign separately to ...