Apply shellcheck fixes to blackhole logic

bin/compile

@@ -127,15 +127,13 @@ done
 # We wanted to `apt-get remove` preinstalled packages, but the root filesystem is read-only.
 # The best we can do is basically make a verbose version of /bin/false, name it the same as the
 # target binary, and have it come before the target binary in the PATH.
-while read BLACKHOLE_LINE; do
-  BINARY_NAME=$(echo "$BLACKHOLE_LINE" | sed -e 's/^blackhole //')
+while read -r BLACKHOLE_LINE; do
+  BINARY_NAME=${BLACKHOLE_LINE/#blackhole /}
   topic "Blackholing $BINARY_NAME"
-  mkdir -p $BUILD_DIR/.apt/usr/bin
+  mkdir -p "$BUILD_DIR"/.apt/usr/bin
   echo 'echo "THIS BINARY HAS BEEN BLACKHOLED"; exit 1' > "$BUILD_DIR/.apt/usr/bin/$BINARY_NAME"
   chmod ugo=rx "$BUILD_DIR/.apt/usr/bin/$BINARY_NAME"
-done < <(
-  cat $BUILD_DIR/Aptfile | grep -s -e '^blackhole '
-)
+done < <(grep -s -e '^blackhole ' "$BUILD_DIR"/Aptfile)
 # END: blackhole packages
 
 topic "Writing profile script"