[PATCH v2] ci: use absolute PYTHON_PATH in the Linux jobs

["SZEDER Gábor" <szeder.dev@gmail.com>]

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>
---

Junio, I see that you picked up the first/RFC version and applied it
on top of v2.26.2.  This patch won't work on v2.26.2, because its 'git
p4' is not compatible with python3 yet.

 ci/lib.sh | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/ci/lib.sh b/ci/lib.sh
index ff24c547c8..3eefec500d 100755
--- a/ci/lib.sh
+++ b/ci/lib.sh
@@ -184,9 +184,9 @@ linux-clang|linux-gcc)
 	if [ "$jobname" = linux-gcc ]
 	then
 		export CC=gcc-8
-		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python3)"
+		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=/usr/bin/python3"
 	else
-		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=$(which python2)"
+		MAKEFLAGS="$MAKEFLAGS PYTHON_PATH=/usr/bin/python2"
 	fi
 
 	export GIT_TEST_HTTPD=true
-- 
2.28.0.rc2.319.g2e16345e2b