CB-634: skip the zsh scrub-report test when /bin/zsh is absent

scrubWritesAnAllowedNofMReport spawned /bin/zsh with no guard, so it
errored on the Gitea CI runner (Linux ARM64 container, no zsh) while
the two sibling zsh tests already skipped there via assumeTrue. Add the
same guard so CI skips instead of failing; the Mac build still runs it.

fleetd/src/test/java/dev/ltms/fleet/member/EnvAllowListScrubTest.java

@@ -92,6 +92,7 @@ class EnvAllowListScrubTest {
     /** The scrub writes its denominator report next to itself; names only, parseable. */
     @Test
     void scrubWritesAnAllowedNofMReport(@TempDir Path tmp) throws Exception {
+        assumeTrue(Files.isExecutable(ZSH), "/bin/zsh not present — nothing to prove here");
         Set<String> allowed = MemberEnvAllowList.derive(List.of());
         Path zdotdir = EnvAllowListScrub.generate(tmp, allowed);