Update check-collector-version.sh: support --dry-run and more

[chalin]

• Closes Update collector-version update script since param has moved #2264
• Updates the file containing the collectorVersion param to be updated (following Make collectorVersion available to entire Collector section #2259)
• Adds support for dry runs via --dry-run flag
• Adds support for forcing latest_version by setting it in the env. that calls the script (useful for testing)
• Changes done in support of running the script from the command line, in particular under macOS

Test the script by running it locally, and it creates the necessary PR, e.g.:

• Bump collector version to 0.70.1 #2265