jenkinsfile: fix the structure

[marquiz]

Jenkins was unhappy with mixing in if-else in the declarative-style
pipeline definition. Try fixing it with imperative style.

#876 did not work as expected:

WorkflowScript: 14: Expected a step @ line 14, column 21.
                       if (env.BRANCH_NAME == 'master') {
                       ^

[codecov-commenter]

Codecov Report

Merging #883 (f224abf) into master (dee7df6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #883   +/-   ##
=======================================
  Coverage   34.92%   34.92%           
=======================================
  Files          60       60           
  Lines        8861     8861           
=======================================
  Hits         3095     3095           
  Misses       5464     5464           
  Partials      302      302           

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