travis-ci: don't run the test suite as root in the 32 bit Linux build

Travis CI runs the 32 bit Linux build job in a Docker container, where
all commands are executed as root by default.  Therefore, ever since
we added this build job in 88dedd5e7 (Travis: also test on 32-bit
Linux, 2017-03-05), we have a bit of code to create a user in the
container matching the ID of the host user and then to run the test
suite as this user.  Matching the host user ID is important, because
otherwise the host user would have no access to any files written by
processes running in the container, notably the logs of failed tests
couldn't be included in the build job's trace log.

Alas, this piece of code never worked, because it sets the variable
holding the user name ($CI_USER) in a subshell, meaning it doesn't
have any effect by the time we get to the point to actually use the
variable to switch users with 'su'.  So all this time we were running
the test suite as root.

Reorganize that piece of code in 'ci/run-linux32-build.sh' a bit to
avoid that problematic subshell and to ensure that we switch to the
right user.  Furthermore, make the script's optional host user ID
option mandatory, so running the build accidentally as root will
become harder when debugging locally.  If someone really wants to run
the test suite as root, whatever the reasons might be, it'll still be
possible to do so by explicitly passing '0' as host user ID.

Finally, one last catch: since commit 7e72cfcee (travis-ci: save prove
state for the 32 bit Linux build, 2017-12-27) the 'prove' test harness
has been writing its state to the Travis CI cache directory from
within the Docker container while running as root.  After this patch
'prove' will run as a regular user, so in future build jobs it won't
be able overwrite a previously written, still root-owned state file,
resulting in build job failures.  To resolve this we should manually
delete caches containing such root-owned files, but that would be a
hassle.  Instead, work this around by changing the owner of the whole
contents of the cache directory to the host user ID.

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

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