jenkins: move .war file from $out to $out/lib/jenkins.war

Fixes #14137, also known as:

  $ nix-shell -p jenkins
  bash: source: /nix/store/ln1yw6c2v8bb2cjqfr1z5aqcssw054wa-jenkins-2.3:
  cannot execute binary file
  [nix-shell exited with error]

The problem is that jenkins.war is not installed inside the directory
$out, but rather _as the file_ $out. Fix it by moving the file to
$out/lib/jenkins.war.

While at it, move buildCommand so that the "meta" section is at the end
of the expression (standard style), and quote shell variables.