runner tests: use /dev/shm tmpfs for stub exec to dodge overlayfs ETXTBSY

setupScript writes a small shell script then immediately fork/execs
it through the runner. The existing write-tmp-then-rename pattern
prevents the userspace ETXTBSY race on a vanilla filesystem, but
overlayfs has an additional window where the kernel's view of
"who holds a writable fd to this inode" can lag the rename — and
the new container-based CI jobs hit it from time to time
("fork/exec ...: text file busy").

Switch the test temp dir to /dev/shm when available (tmpfs has
no overlay layering), falling back to t.TempDir() when /dev/shm
isn't usable. Production code path is unaffected; this is a
pure test helper change.

internal/agent/runner/runner_test.go

@@ -50,9 +50,16 @@ func (s *fakeSender) snapshot() []api.Envelope {
 // returns ETXTBSY ("text file busy"). Once the rename lands, the
 // final path is a fresh dirent pointing at an inode that has no
 // writable fd open anywhere — exec is safe.
+//
+// Temp dirs live under /dev/shm when it's writable (tmpfs). On a
+// containerised CI runner, t.TempDir() lands on overlayfs, which
+// has its own ETXTBSY surface that the rename pattern above does
+// not fully close — the kernel's "writable inode" bookkeeping
+// can lag the userspace close on overlay's upper layer. Falling
+// back to a real tmpfs sidesteps the problem entirely.
 func setupScript(t *testing.T, body string) string {
 	t.Helper()
-	dir := t.TempDir()
+	dir := tempDirForExec(t)
 	final := filepath.Join(dir, "restic")
 	tmp := final + ".tmp"
 	if err := os.WriteFile(tmp, []byte("#!/bin/sh\n"+body+"\n"), 0o755); err != nil {
@@ -64,6 +71,21 @@ func setupScript(t *testing.T, body string) string {
 	return final
 }
 
+// tempDirForExec returns a per-test temp dir that's safe to write
+// then exec from. Uses /dev/shm (tmpfs) when available, otherwise
+// falls back to t.TempDir(). See the setupScript comment for why.
+func tempDirForExec(t *testing.T) string {
+	t.Helper()
+	if st, err := os.Stat("/dev/shm"); err == nil && st.IsDir() {
+		dir, err := os.MkdirTemp("/dev/shm", "rmrunner-")
+		if err == nil {
+			t.Cleanup(func() { _ = os.RemoveAll(dir) })
+			return dir
+		}
+	}
+	return t.TempDir()
+}
+
 // firstEnvOfType returns the first envelope with the given type, or
 // fails the test if none is found.
 func firstEnvOfType(t *testing.T, envs []api.Envelope, mt api.MessageType) api.Envelope {