Pants should be able to tell if its launcher script is up-to-date

[chrisjrn]

Watching the back-and-forth on #17563, we're running into an issue with the VERSION file again. We didn't change it in the first place because the path to the VERSION file is hardcoded into the launcher script.

Moving the VERSION file has been assessed to have minimal impact, because it only affects users of PANTS_FROM_SHA, however, with future changes (such as adding new distribution formats), it would be useful to alert users to the launcher script changing, in case the breakage is more severe.

[thejcannon]

Just hardcoding a "version" into the launcher and then the latest "version" in pants itself might be sufficient.

Incorrect for older pants versions, but dead simple

[huonw]

After pantsbuild/setup#129, the setup script now has SCRIPT_VERSION, but this isn't pushed into the pants invocation (and presumably should be propagated with a more specific name like PANTS_BOOTSTRAP_SCRIPT_VERSION or something).