Apply forceEnv option to Babel transformation

[chrisvasz]

As per the discussion in #368, this PR applies the forceEnv option to the Babel transformation.

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Bug fix

What is the current behavior? (You can also link to an open issue here)
As discussed here, the forceEnv option applies to the cacheIdentifier, but does not actually apply to the Babel transformation.

What is the new behavior?
The forceEnv option now applies to the Babel transformation as well.

Does this PR introduce a breaking change?

• No

[codecov-io]

Codecov Report

Merging #379 into master will increase coverage by 1.56%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #379      +/-   ##
==========================================
+ Coverage   81.87%   83.43%   +1.56%     
==========================================
  Files           6        6              
  Lines         160      163       +3     
  Branches       35       35              
==========================================
+ Hits          131      136       +5     
+ Misses         13       12       -1     
+ Partials       16       15       -1

Impacted Files	Coverage Δ
src/index.js	86.3% <100%> (+3.44%)	✅

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[tleunen]

What about with the cacheDirectory flag?

[chrisvasz]

What about it? I tried to think of how cacheDirectory would factor into this change, but I couldn't think of a way immediately. cacheIdentifier definitely makes sense.

[tleunen]

process.env.BABEL_ENV is still not set when cacheDirectory is true.
The transpilation is done inside the cache. See transform: transpile.