Fix signing task in build.gradle

[lbergelson]

• Fix the signing task configuration in build.gradle so that releases are signed.
  I had to do this for the 2.19.0 release.

Checklist

• Code compiles correctly
• New tests covering changes and new functionality
• All tests passing
• Extended the README / documentation, if necessary
• Is not backward compatible (breaks binary or source compatibility)

[cmnbroad]

Seems fine as far as I can tell.

[codecov-io]

Codecov Report

Merging #1325 into master will increase coverage by 0.037%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##              master    #1325       +/-   ##
==============================================
+ Coverage     67.793%   67.83%   +0.037%     
- Complexity      8251     8268       +17     
==============================================
  Files            562      562               
  Lines          33639    33699       +60     
  Branches        5641     5667       +26     
==============================================
+ Hits           22805    22858       +53     
+ Misses          8659     8658        -1     
- Partials        2175     2183        +8

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/htsjdk/variant/bcf2/BCF2Codec.java	78.947% <0%> (+1.566%)	51% <0%> (+16%)	⬆️
...samtools/util/AsyncBlockCompressedInputStream.java	76% <0%> (+4%)	13% <0%> (+1%)	⬆️