upstream changes to unixStartScript.txt

[keturn]

Someone with a lot of respect for POSIX sh overhauled the unixStartScript.txt template for gradle 7.2: https://github.com/gradle/gradle/pull/17666/files#diff-a549a2629a3d96f76e3f043a1526e48fb086e90e9d9d03ae0f7abf6d3e803333

I guess now that we're not trying to apply a diff to upstream's copy, we can address this independently of any gradle upgrade, as the names and values of the template variables did not change in that PR.

But at some point we will want to rebase our script on that one so that other future changes to upstream will make sense.