cc: Use parameter expansion instead of basename (#24509)

While debugging #24508, I noticed that we call `basename` in `cc`. The
same can be achieved by using Bash's parameter expansion, saving one
external process per call.

Parameter expansion cannot replace basename for directories in some
cases, but is guaranteed to work for executables.

lib/spack/env/cc

@@ -128,7 +128,7 @@ fi
 #    ld      link
 #    ccld    compile & link
 
-command=$(basename "$0")
+command="${0##*/}"
 comp="CC"
 case "$command" in
     cpp)