Fix incorrect evaluation whether PR is prerelease

[Patrik-Stas]

2 changes:

• previously every non-master PR was marked as pre-release, even if it did not contain pre-release label. This is because github expression are not directly understood by bash.
• pull request with bumped version is not understood as pre-release. The only way to pre-release is by assigning pre-release label

[codecov-io]

Codecov Report

Merging #244 (a2ce8be) into master (71237f3) will decrease coverage by 5.71%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
- Coverage   57.38%   51.66%   -5.72%     
==========================================
  Files         149      148       -1     
  Lines       20009    19080     -929     
  Branches     4502     4365     -137     
==========================================
- Hits        11482     9858    -1624     
- Misses       5178     6158     +980     
+ Partials     3349     3064     -285     

Flag	Coverage Δ
integration	?
unittests	51.66% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
libvcx/src/libindy/utils/anoncreds.rs	21.49% <0.00%> (-59.16%)	⬇️
libvcx/src/libindy/utils/pool.rs	17.39% <0.00%> (-58.66%)	⬇️
libvcx/src/utils/plugins.rs	0.00% <0.00%> (-42.86%)	⬇️
libvcx/src/libindy/utils/ledger.rs	11.74% <0.00%> (-39.24%)	⬇️
libvcx/src/aries/mod.rs	1.04% <0.00%> (-30.77%)	⬇️
libvcx/src/utils/random.rs	75.00% <0.00%> (-25.00%)	⬇️
libvcx/src/utils/devsetup.rs	26.70% <0.00%> (-21.53%)	⬇️
libvcx/src/libindy/utils/payments.rs	42.98% <0.00%> (-21.15%)	⬇️
libvcx/src/libindy/proofs/verifier/verifier.rs	70.37% <0.00%> (-18.06%)	⬇️
...aries/handlers/proof_presentation/prover/prover.rs	57.79% <0.00%> (-17.32%)	⬇️
... and 27 more

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