ci: fixup task runner chroot test

This PR is 2 fixes for the flaky TestTaskRunner_TaskEnv_Chroot test.

And also the TestTaskRunner_Download_ChrootExec test.

- Use TinyChroot to stop copying gigabytes of junk, which causes GHA
to fail to create the environment in time.

- Pre-create cgroups on V2 systems. Normally the cgroup directory is
managed by the cpuset manager, but that is not active in taskrunner tests,
so create it by hand in the test framework.

ci/slow.go

@@ -26,3 +26,41 @@ func Parallel(t *testing.T) {
 		t.Parallel()
 	}
 }
+
+// TinyChroot is useful for testing, where we do not use anything other than
+// trivial /bin commands like sleep and sh. Copying a minimal chroot helps in
+// environments like GHA with very poor [network] disk performance.
+//
+// Note that you cannot chroot a symlink.
+//
+// Do not modify this value.
+var TinyChroot = map[string]string{
+	// destination: /bin
+	"/usr/bin/sleep": "/bin/sleep",
+	"/usr/bin/dash":  "/bin/sh",
+	"/usr/bin/bash":  "/bin/bash",
+	"/usr/bin/cat":   "/bin/cat",
+
+	// destination: /usr/bin
+	"/usr/bin/stty":   "/usr/bin/stty",
+	"/usr/bin/head":   "/usr/bin/head",
+	"/usr/bin/mktemp": "/usr/bin/mktemp",
+	"/usr/bin/echo":   "/usr/bin/echo",
+	"/usr/bin/touch":  "/usr/bin/touch",
+	"/usr/bin/stat":   "/usr/bin/stat",
+
+	// destination: /etc/
+	"/etc/ld.so.cache":  "/etc/ld.so.cache",
+	"/etc/ld.so.conf":   "/etc/ld.so.conf",
+	"/etc/ld.so.conf.d": "/etc/ld.so.conf.d",
+	"/etc/passwd":       "/etc/passwd",
+	"/etc/resolv.conf":  "/etc/resolv.conf",
+
+	// others
+	"/lib":                 "/lib",
+	"/lib32":               "/lib32",
+	"/lib64":               "/lib64",
+	"/usr/lib/jvm":         "/usr/lib/jvm",
+	"/run/resolvconf":      "/run/resolvconf",
+	"/run/systemd/resolve": "/run/systemd/resolve",
+}