ci: use absolute PYTHON_PATH in the Linux jobs

ci: use absolute PYTHON_PATH in the Linux jobs

In our test suite, when 'git p4' invokes a Git command as a
subprocesses, then it should run the 'git' binary we are testing.
Unfortunately, this is not the case in the 'linux-clang' and
'linux-gcc' jobs on Travis CI, where 'git p4' runs the system
'/usr/bin/git' instead.

Travis CI's default Linux image includes 'pyenv', and all Python
invocations that involve PATH lookup go through 'pyenv', e.g. our
'PYTHON_PATH=$(which python3)' sets '/opt/pyenv/shims/python3' as
PYTHON_PATH, which in turn will invoke '/usr/bin/python3'.  Alas, the
'pyenv' version included in this image is buggy, and prepends the
directory containing the Python binary to PATH even if that is a
system directory already in PATH near the end.  Consequently, 'git p4'
in those jobs ends up with its PATH starting with '/usr/bin', and then
runs '/usr/bin/git'.

So use the absolute paths '/usr/bin/python{2,3}' explicitly when
setting PYTHON_PATH in those Linux jobs to avoid the PATH lookup and
thus the bogus 'pyenv' from interfering with our 'git p4' tests.
Don't bother with special-casing Travis CI: while this issue doesn't
affect the corresponding Linux jobs on GitHub Actions, both CI systems
use Ubuntu LTS-based images, so we can safely rely on these Python
paths.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>