shellcheck: fix lint of scripts in /helper

helper/update-fixed-output-derivation.sh

@@ -7,10 +7,10 @@ flakeOutput=$2
 # A pattern in a line preceding the hash that should be updated
 patternPrecedingHash=$3
 
-sed -i "/$patternPrecedingHash/,/hash/ s|hash = .*|hash = \"\";|" $file
+sed -i "/$patternPrecedingHash/,/hash/ s|hash = .*|hash = \"\";|" "$file"
 # Display stderr and capture it. stdbuf is required to disable output buffering.
 stderr=$(
-    nix build --no-link -L .#$flakeOutput |&
+    nix build --no-link -L ".#$flakeOutput" |&
       stdbuf -oL grep -v '\berror:.*failed to build$' |
       tee /dev/stderr || :
 )
@@ -20,5 +20,5 @@ if [[ ! $hash ]]; then
     echo "Error: No hash in build output."
     exit 1
 fi
-sed -i "/$patternPrecedingHash/,/hash/ s|hash = .*|hash = \"$hash\";|" $file
+sed -i "/$patternPrecedingHash/,/hash/ s|hash = .*|hash = \"$hash\";|" "$file"
 echo "(Note: The above hash mismatch message is not an error. It is part of the fetching process.)"