Use environment variable to detect publishing state.

[artem-zinnatullin]

Uhh damn Gradle, Travis and everything.

I wasn't able to use Gradle task graph analysis, so one way is to use env variable as I do here, another way is to always try to read release-related env variables for signing but I do find it more error prone and hard to debug in case if something will go wrong, current process will break build asap which is easier to debug

Sorry for so many PRs, but it's hard to test locally because CI has pretty different setup

[codecov-io]

Codecov Report

Merging #834 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #834   +/-   ##
=========================================
  Coverage     97.41%   97.41%           
  Complexity      791      791           
=========================================
  Files            91       91           
  Lines          2707     2707           
  Branches        305      305           
=========================================
  Hits           2637     2637           
  Misses           38       38           
  Partials         32       32

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