Allow perf script to find a script in the exec path.

Example:

Before:

 $ perf record -a -e intel_pt/branch=0/ sleep 0.1
 [ perf record: Woken up 1 times to write data ]
 [ perf record: Captured and wrote 0.954 MB perf.data ]
 $ perf script intel-pt-events.py 2>&1 | head -3
   Error: Couldn't find script `intel-pt-events.py'
   See perf script -l for available scripts.
 $ perf script -s intel-pt-events.py 2>&1 | head -3
 Can't open python script "intel-pt-events.py": No such file or directory
 $ perf script ~/libexec/perf-core/scripts/python/intel-pt-events.py 2>&1 | head -3
   Error: Couldn't find script `/home/ahunter/libexec/perf-core/scripts/python/intel-pt-events.py'
   See perf script -l for available scripts.
 $

After:

 $ perf script intel-pt-events.py 2>&1 | head -3
 Intel PT Power Events and PTWRITE
            perf  8123/8123  [000]       551.230753986     cbr:  42  freq: 4219 MHz  (156%)                0 [unknown] ([unknown])
            perf  8123/8123  [001]       551.230808216     cbr:  42  freq: 4219 MHz  (156%)                0 [unknown] ([unknown])
 $ perf script -s intel-pt-events.py 2>&1 | head -3
 Intel PT Power Events and PTWRITE
            perf  8123/8123  [000]       551.230753986     cbr:  42  freq: 4219 MHz  (156%)                0 [unknown] ([unknown])
            perf  8123/8123  [001]       551.230808216     cbr:  42  freq: 4219 MHz  (156%)                0 [unknown] ([unknown])
 $ perf script ~/libexec/perf-core/scripts/python/intel-pt-events.py 2>&1 | head -3
 Intel PT Power Events and PTWRITE
            perf  8123/8123  [000]       551.230753986     cbr:  42  freq: 4219 MHz  (156%)                0 [unknown] ([unknown])
            perf  8123/8123  [001]       551.230808216     cbr:  42  freq: 4219 MHz  (156%)                0 [unknown] ([unknown])
 $
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Link: http://lore.kernel.org/lkml/20210524065718.11421-1-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>