Minor cleanups: gitignore out/, expect() messages, comment wording

- .gitignore: ignore the /out/ ISO build dir
- bos-settings: use expect() with messages over unwrap() for piped stdio;
  drop a stray blank line
- pacman.conf: reword the SigLevel=Never TODO as a future-improvement note

.gitignore

@@ -27,6 +27,7 @@ secrets/
 # archiso build artifacts (these are large and reproducible)
 /iso-build/
 /iso-out/
+/out/
 *.iso
 *.img