build_all_repos: Use more bash-isms, address issues flagged by shellcheck

[jrha]

• Use $* instead of $@ when concatenating with a string.
• Declare and export environment variables separately.
• Double quote array expansions, otherwise they're like $* and break on spaces.
• Double quote lots of variables to prevent globbing and word splitting.
• Remove unnecessary from $/${} arithmetic variables.
• Use globs rather than iterating over ls output.
• Quote parameters to tr to prevent glob expansion.
• Use ${variable//search/replace} instead of piping to sed where possible.
• Use $(…) instead of legacy `…`.
• Use [[ ]] rather than [ ] to reduce number of sub-processes.
• Fail script hard if an attempt to change directory fails.
• Remove some stray white-space.
• Ensure that curl and sudo curlrender correctly in wrapped output.

[jrha]

This is the last of the build_all_repos clean-up work.

[stdweird]

@jrha thx for this