ci: restore running httpd tests

ci: restore running httpd tests

Once upon a time GIT_TEST_HTTPD was a tristate variable and we
exported 'GIT_TEST_HTTPD=YesPlease' in our CI scripts to make sure
that we run the httpd tests in the Linux Clang and GCC build jobs, or
error out if they can't be run for any reason [1].

Then 3b072c577b (tests: replace test_tristate with "git env--helper",
2019-06-21) came along, turned GIT_TEST_HTTPD into a bool, but forgot
to update our CI scripts accordingly.  So, since GIT_TEST_HTTPD is set
explicitly, but its value is not one of the standardized true values,
our CI jobs have been simply skipping the httpd tests in the last
couple of weeks.

Set 'GIT_TEST_HTTPD=true' to restore running httpd tests in our CI
jobs.

[1] a1157b76eb (travis-ci: set GIT_TEST_HTTPD in 'ci/lib-travisci.sh',
    2017-12-12)

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