[now-static-build] Print version of static generator during build

[styfle]

If an environment variable such as HUGO_VERSION is used, we want to always render the version that we used in the logs.

If no such env var is used, we don’t want to log anything.

PRODUCT-132 #close

[codecov-io]

Codecov Report

Merging #3068 into canary will increase coverage by 1.86%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           canary    #3068      +/-   ##
==========================================
+ Coverage    11.6%   13.47%   +1.86%     
==========================================
  Files         267      267              
  Lines       10069    10301     +232     
  Branches     1299     1212      -87     
==========================================
+ Hits         1169     1388     +219     
- Misses       8809     8851      +42     
+ Partials       91       62      -29

Impacted Files	Coverage Δ
src/util/metrics.ts	68.18% <0%> (-2.41%)	⬇️
src/util/projects/get-project-by-id-or-name.ts	28.57% <0%> (+3.57%)	⬆️
src/util/get-files.ts	91.66% <0%> (+3.88%)	⬆️
src/util/dev/router.ts	82.69% <0%> (+4.12%)	⬆️
src/util/dev/builder.ts	76.92% <0%> (+4.4%)	⬆️
src/util/dev/yarn-installer.ts	90.74% <0%> (+5.02%)	⬆️
src/util/dev/server.ts	66.43% <0%> (+5.41%)	⬆️
src/util/prefer-v2-deployment.ts	61.9% <0%> (+5.65%)	⬆️
src/util/output/create-output.ts	43.58% <0%> (+12.33%)	⬆️

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