CI: Check for syntax errors or linting issues in Linux/macOS build script

[guihkx]

• CI: Check/lint the build script with bash -n and shellcheck before trying to build RSS Guard.
• Fix warnings given by shellcheck.
• Use set -e throughout the script to make errors more obvious.
• Introduce the $app_id variable (Linux-only), and use it where appropriate, to reduce code duplication.
• Use the already existing $prefix variable where appropriate, to reduce code duplication.
• Use an array instead of a string to build the GStreamer paths for linuxdeployqt.

[guihkx]

An example of how the shellcheck step can look like:

If you think it's going to be more annoying than helpful, I can remove it.

[martinrotter]

This is to be merged AFTER 4.2.6 release too right?

[guihkx]

No, this can be merged right away (if you want). They're mostly cosmetic changes.

[guihkx]

I'd actually prefer if you could merge this one before #836.