test: write-then-rename script-bin helpers (avoid ETXTBSY under -race)

CI run #48 failed with:

  --- FAIL: TestRunInitShipsStartedAndFinished
      RunInit: ... fork/exec /tmp/.../restic: text file busy

setupScript and setupScriptBin used os.WriteFile to write a shell
script directly at the final path, then exec'd it. Under -race +
many t.Parallel tests, a fork-from-another-goroutine could inherit
the still-open writable fd from one of those WriteFile calls; the
kernel returns ETXTBSY when the freshly-execed binary still has a
writable fd anywhere on the system.

Fix: write to "<path>.tmp", then os.Rename into place. The rename
is a pure dirent op; by the time the final path exists, no process
has a writable fd on its inode and exec is safe. -race + -count=5
on both runner packages now passes consistently.

internal/agent/runner/runner_test.go

@@ -20,14 +20,26 @@ func (s *fakeSender) Send(e api.Envelope) error {
 
 // setupScript writes a shell script (without shebang) to a temp dir,
 // names it "restic", makes it executable, and returns the path.
+//
+// Writes to "<path>.tmp" then renames into place. The rename is what
+// makes this race-free: under -race + many t.Parallel tests, a
+// fork-from-another-goroutine can inherit the writable fd from
+// os.WriteFile before close completes, and exec'ing the file then
+// 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.
 func setupScript(t *testing.T, body string) string {
 	t.Helper()
 	dir := t.TempDir()
-	p := filepath.Join(dir, "restic")
-	if err := os.WriteFile(p, []byte("#!/bin/sh\n"+body+"\n"), 0o755); err != nil {
-		t.Fatalf("setupScript: %v", err)
+	final := filepath.Join(dir, "restic")
+	tmp := final + ".tmp"
+	if err := os.WriteFile(tmp, []byte("#!/bin/sh\n"+body+"\n"), 0o755); err != nil {
+		t.Fatalf("setupScript: write tmp: %v", err)
 	}
-	return p
+	if err := os.Rename(tmp, final); err != nil {
+		t.Fatalf("setupScript: rename: %v", err)
+	}
+	return final
 }
 
 // firstEnvOfType returns the first envelope with the given type, or