Making watch reload the build script if it changes

[jakubDoka]

This should resolve #20602.

I have tested this manually and it works. I have the following questions:

• Do we have any automated tests that can test this?
• Is the way I reload on the build script the most efficient way?
• Is it fine for the watch to end when there is an error in the build script?
• Of I resolve the build script path correctly (are there any edge cases that would break this)

[alexrp]

Would this trigger a rebuild if a file that build.zig imports changes?