scons: Replace find_first_prog() with the built in Detect().

scons: Replace find_first_prog() with the built in Detect().

The built in environment method Detect() does the same thing, that is it
finds the first program available from a list of options.

Change-Id: I3763ae5cc9dd22ee322908c0a7a2c037dc91d5a5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27126
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>