The HTML page changed and the update script was no longer finding the latest version.
I changed it to query for the latest version using dedicated URL:
https://forum.sublimetext.com/t/fixed-download-url-for-latest-version/23763/7
Also terminated the update sooner if we detect no update available.
update-source-version handles that normally but we are cleaning the version
so that we can update both platforms at the same time.
Whenever we create scripts that are installed to $out, we must use runtimeShell
in order to get the shell that can be executed on the machine we create the
package for. This is relevant for cross-compiling. The only use case for
stdenv.shell are scripts that are executed as part of the build system.
Usages in checkPhase are borderline however to decrease the likelyhood
of people copying the wrong examples, I decided to use runtimeShell as well.
