Merge #17361: script: Lint Gitian descriptors with ShellCheck

17f81e9648 script: Enable SC2001 rule for Gitian scripts (Hennadii Stepanov)
61bb21b418 script: Enable SC2155 rule for Gitian scripts (Hennadii Stepanov)
577682d9e8 script: Enable SC2006 rule for Gitian scripts (Hennadii Stepanov)
14aded46df script: Lint Gitian descriptors with ShellCheck (Hennadii Stepanov)

Pull request description:

  This PR extracts shell scripts from Gitian descriptors (`contrib/gitian-descriptors/`) and checks for ShellCheck warnings as any other one.

  Some non-controversial warnings are fixed.

ACKs for top commit:
  practicalswift:
    ACK 17f81e9648 -- diff looks correct

Tree-SHA512: bdfa3d35bbb65ff634c90835d75c3df63e958b558599771d21366724f5cf64da83a68957d926e926a99c3704b9529e96a17697dc8d9ff3adf7154d9cb1999a8d

ci/lint/04_install.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 #
-# Copyright (c) 2018 The Bitcoin Core developers
+# Copyright (c) 2018-2019 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -8,6 +8,7 @@ export LC_ALL=C
 
 travis_retry pip3 install codespell==1.15.0
 travis_retry pip3 install flake8==3.7.8
+travis_retry pip3 install yq
 
 SHELLCHECK_VERSION=v0.6.0
 curl -s "https://storage.googleapis.com/shellcheck/shellcheck-${SHELLCHECK_VERSION}.linux.x86_64.tar.xz" | tar --xz -xf - --directory /tmp/