Merge #16327: scripts and tools: Update ShellCheck linter

1ac454a384 Enable ShellCheck rules (Hennadii Stepanov)

Pull request description:

  Enable some simple ShellCheck rules.

  Note for reviewers: `bash` and `shellcheck` on macOS are different from ones on Ubuntu.
  For local tests the latest `shellcheck` version 0.6.0 should be used (see #15166).

ACKs for top commit:
  practicalswift:
    utACK 1ac454a384
  dongcarl:
    utACK 1ac454a
  fanquake:
    ACK 1ac454a384

Tree-SHA512: 8d0a3a5c09fe1a0c22120178f5e6b80f81f746f8c3356b7701ff301c117acb2edea8fe08f08fb54ed73f94b1617515fb239fa28e7ab4121f74872e6494b6f20e

src/qt/res/movies/makespinner.sh

@@ -9,6 +9,6 @@ FRAMEDIR=$(dirname $0)
 for i in {0..35}
 do
     frame=$(printf "%03d" $i)
-    angle=$(($i * 10))
+    angle=$((i * 10))
     convert $FRAMEDIR/../src/spinner.png -background "rgba(0,0,0,0.0)" -distort SRT $angle $FRAMEDIR/spinner-$frame.png
 done