Make readonly bash variables explicitly readonly

[mtlynch]

Description

In bash scripts, I noticed a few variables that are read-only, but they're not declared as readonly in bash. Declaring them as readonly protects against potential bugs in the future where code may unintentionally overwrite these variables.

I didn't apply readonly to the variable PACKAGE_VERSION because it is unused and I'm proposing its deletion in #2666

Corresponding issue: N/A

Testing

Please explain how you tested this change manually, and, if applicable, what new tests you added. If
you're making a change to just the website, you can omit this section.

I ran the scripts manually.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[alexpetros]

Closing per contributions guides (this is not a bugfix, documentation update, or solicited new feature). These are small scripts, it's not that big a deal