$srctree is defined by kbuild in the toplevel Makefile.

How is this variable passed to the file “scripts/nsdeps”?

quoted

* Would you like to support a separate build directory for desired adjustments?

No, as the purpose of this script is to directly patch the kernel
sources where applicable.

Will there occasionally be a need to provide a generated patch
(without in-place file modification)?

quoted

I suggest to assign the name for the temporary file to a variable
which should be used by subsequent commands.

I somehow don't agree that this is an improvement to the code as the
variable would likely be something like ${source_file_tmp}.

Would you dare to choose a shorter variable name?

${source_file}.tmp does express the intent of a temporary file next to
the original source file and the reader of the code does not need to
reason about the value of ${source_file_tmp}.

I would find a code variant with less suffix repetition nicer.

Regards,
Markus