test: fix mpv tests on systems without /bin/bash installed - 4301 (#4302)

Not all systems have bash at `/bin/bash`. `/bin/sh` is POSIX and should
be present on all systems making this much more portable. No bash
features are currently used in the script so this change should be safe.

core/playback/mpv/mpv_test.go

@@ -372,7 +372,7 @@ goto loop
 `
 	} else {
 		scriptExt = ".sh"
-		scriptContent = `#!/bin/bash
+		scriptContent = `#!/bin/sh
 echo "$0"
 for arg in "$@"; do
     echo "$arg"