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/restic/runner_test.go

@@ -13,15 +13,23 @@ import (
 // makes it executable, and returns its path. scriptBody is the
 // complete script content (without the shebang line — that's added
 // automatically).
+// Writes to "<path>.tmp" then renames into place — see the matching
+// helper in internal/agent/runner/runner_test.go for the ETXTBSY
+// race rationale. Same fix applied here so this helper doesn't lose
+// the race the next time CI gets unlucky.
 func setupScriptBin(t *testing.T, scriptBody string) string {
 	t.Helper()
 	dir := t.TempDir()
-	p := filepath.Join(dir, "restic")
+	final := filepath.Join(dir, "restic")
+	tmp := final + ".tmp"
 	content := "#!/bin/sh\n" + scriptBody + "\n"
-	if err := os.WriteFile(p, []byte(content), 0o755); err != nil {
-		t.Fatalf("setupScriptBin: %v", err)
+	if err := os.WriteFile(tmp, []byte(content), 0o755); err != nil {
+		t.Fatalf("setupScriptBin: write tmp: %v", err)
 	}
-	return p
+	if err := os.Rename(tmp, final); err != nil {
+		t.Fatalf("setupScriptBin: rename: %v", err)
+	}
+	return final
 }
 
 // captureLines returns a LineHandler that appends "stream:line" into