test: Make more shell scripts verifiable by the shellcheck tool

contrib/guix/libexec/prelude.bash

@@ -2,10 +2,10 @@
 export LC_ALL=C
 set -e -o pipefail
 
-# shellcheck source=../../shell/realpath.bash
+# shellcheck source=contrib/shell/realpath.bash
 source contrib/shell/realpath.bash
 
-# shellcheck source=../../shell/git-utils.bash
+# shellcheck source=contrib/shell/git-utils.bash
 source contrib/shell/git-utils.bash
 
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