fmf: Use correct os-release

With commit 2e8b932e, run-test.sh now runs inside the cockpit/tasks
container, and its os-release is irrelevant. Read the host's os-release
instead to compute a correct `$TEST_OS`.

test/browser/browser.sh

@@ -36,5 +36,6 @@ exec podman \
         --security-opt=label=disable \
         --volume="${TMT_TEST_DATA}":/logs:rw,U --env=LOGS=/logs \
         --volume="$(pwd)":/source:rw,U --env=SOURCE=/source \
+        --volume=/usr/lib/os-release:/run/host/usr/lib/os-release:ro \
         "${CONTAINER}" \
             sh /source/test/browser/run-test.sh

test/browser/run-test.sh

@@ -20,7 +20,7 @@ fi
 # disable detection of affected tests; testing takes too long as there is no parallelization
 mv .git dot-git
 
-. /etc/os-release
+. /run/host/usr/lib/os-release
 export TEST_OS="${ID}-${VERSION_ID/./-}"
 
 if [ "${TEST_OS#centos-}" != "$TEST_OS" ]; then