travis-ci: move setting environment variables to 'ci/lib-travisci.sh'

travis-ci: move setting environment variables to 'ci/lib-travisci.sh'

Our '.travis.yml's 'env.global' section sets a bunch of environment
variables for all build jobs, though none of them actually affects all
build jobs.  It's convenient for us, and in most cases it works just
fine, because irrelevant environment variables are simply ignored.

However, $GIT_SKIP_TESTS is an exception: it tells the test harness to
skip the two test scripts that are prone to occasional failures on
OSX, but as it's set for all build jobs those tests are not run in any
of the build jobs that are capable to run them reliably, either.

Therefore $GIT_SKIP_TESTS should only be set in the OSX build jobs,
but those build jobs are included in the build matrix implicitly (i.e.
by combining the matrix keys 'os' and 'compiler'), and there is no way
to set an environment variable only for a subset of those implicit
build jobs.  (Unless we were to add new scriptlets to '.travis.yml',
which is exactly the opposite direction that we took with commit
657343a60 (travis-ci: move Travis CI code into dedicated scripts,
2017-09-10)).

So move setting $GIT_SKIP_TESTS to 'ci/lib-travisci.sh', where it can
trivially be set only for the OSX build jobs.

Furthermore, move setting all other environment variables from
'.travis.yml' to 'ci/lib-travisci.sh', too, because a couple of
environment variables are already set there, and this way all
environment variables will be set in the same place.  All the logic
controlling our builds is already in the 'ci/*' scripts anyway, so
there is really no good reason to keep the environment variables
separately.

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