scripts: Address shellcheck issues

Report issues by shell files with:

    git ls-files '*.sh' | xargs shellcheck --exclude=SC2162

Address the following:

- SC1087: Use braces when expanding arrays
- SC1091: Not following
- SC2004: `$`/`${}` is unnecessary on arithmetic variables
- SC2024: `sudo` doesn't affect redirects
- SC2034: foo appears unused. Verify it or export it
- SC2086: Double quote to prevent globbing and word splitting
- SC2087: Quote `EOF`
- SC2115: Use `"${var:?}"` to ensure this never expands to `/*`
- SC2148: Add a shebang

Addresses (at least partially) some POSIX/dash issues:

- SC2113: `function` keyword is non-standard
- SC3010: In POSIX sh, `[[` `]]` is undefined
- SC3014: In POSIX sh, `==` in place of `=` is undefined
- SC3020: In POSIX sh, `&>` is undefined
- SC3046: In POSIX sh, `source` in place of `.` is undefined

Does not address:

- SC2162: `read` without `-r` will mangle backslashes
- Any other POSIX/dash-specific issues

Signed-off-by: Tim Crawford <tcrawford@system76.com>

scripts/install-deps.sh

@@ -1,15 +1,17 @@
 #!/usr/bin/env bash
 # SPDX-License-Identifier: GPL-3.0-only
 
+# shellcheck disable=SC1091
+
 set -eE
 
-function msg {
+msg() {
   echo -e "\x1B[1m$*\x1B[0m" >&2
 }
 
 trap 'msg "\x1B[31mFailed to install dependencies!"' ERR
 
-source /etc/os-release
+. /etc/os-release
 
 msg "Installing system build dependencies"
 if [[ "${ID}" =~ "debian" ]] || [[ "${ID_LIKE}" =~ "debian" ]]; then