test: skip the symlink part of test_touch_file() in GH Actions

Our (RHEL 8) touch_file() is not clever enough and does chmod() on a
symlink, which fails with EOPNOTSUPP on newer kernels. This is not an
issue on the RHEL 8 kernel, where doing chmod() on a symlink works
(albeit only on tmpfs) but in GH Actions we run in a container, and with
the underlying kernel doing chmod() on a symlink fails even on tmpfs:

RHEL 8:
~# mount -t tmpfs tmpfs /tmp
~# (cd /tmp; ln -s symlink dangling; ln -s /etc/os-release symlink)
~# (cd /var/tmp; ln -s symlink dangling; ln -s /etc/os-release symlink)
~# gcc -o main main.c -D_GNU_SOURCE
~# ./main /tmp/dangling
chmod(/proc/self/fd/3)=0 (0)
~# ./main /tmp/symlink
chmod(/proc/self/fd/3)=0 (0)
~# ./main /var/tmp/dangling
chmod(/proc/self/fd/3)=-1 (95)
~# ./main /var/tmp/symlink
chmod(/proc/self/fd/3)=-1 (95)

Newer kernel:
~# uname -r
6.7.4-200.fc39.x86_64
~# ./main /tmp/dangling
chmod(/proc/self/fd/3)=-1 (95)
~# ./main /tmp/symlink
chmod(/proc/self/fd/3)=-1 (95)
~# ./main /var/tmp/dangling
chmod(/proc/self/fd/3)=-1 (95)
~# ./main /var/tmp/symlink
chmod(/proc/self/fd/3)=-1 (95)

Backporting the necessary patches would be way too risky so late in the
RHEL 8 cycle, so let's just skip the offending test when running in GH
Actions. To do that we have to jump through a couple of hoops, since
RHEL 8 systemd can't detect docker. Oh well.

See: #434

RHEL-only

src/test/test-fs-util.c

@@ -15,6 +15,7 @@
 #include "stdio-util.h"
 #include "string-util.h"
 #include "strv.h"
+#include "tests.h"
 #include "user-util.h"
 #include "util.h"
 #include "virt.h"
@@ -544,15 +545,17 @@ static void test_touch_file(void) {
                 assert_se(timespec_load(&st.st_mtim) == test_mtime);
         }
 
-        a = strjoina(p, "/lnk");
-        assert_se(symlink("target", a) >= 0);
-        assert_se(touch_file(a, false, test_mtime, test_uid, test_gid, 0640) >= 0);
-        assert_se(lstat(a, &st) >= 0);
-        assert_se(st.st_uid == test_uid);
-        assert_se(st.st_gid == test_gid);
-        assert_se(S_ISLNK(st.st_mode));
-        assert_se((st.st_mode & 0777) == 0640);
-        assert_se(timespec_load(&st.st_mtim) == test_mtime);
+        if (!streq_ptr(ci_environment(), "github-actions")) {
+                a = strjoina(p, "/lnk");
+                assert_se(symlink("target", a) >= 0);
+                assert_se(touch_file(a, false, test_mtime, test_uid, test_gid, 0640) >= 0);
+                assert_se(lstat(a, &st) >= 0);
+                assert_se(st.st_uid == test_uid);
+                assert_se(st.st_gid == test_gid);
+                assert_se(S_ISLNK(st.st_mode));
+                assert_se((st.st_mode & 0777) == 0640);
+                assert_se(timespec_load(&st.st_mtim) == test_mtime);
+        }
 }
 
 static void test_unlinkat_deallocate(void) {