feat: s2i builder env var interpolation

[lkingland]

• 🎁 S2I Builder Env Interpolation

More specifically:

• Extracts env var interpolation logic from buildpack builder to shared package static fn.Interpolate
• Refactors implementation slightly to pass-through unmatched values (previously a bit ambiguous, presuming permissive).
• Adds interpolation tests, including explicit tests for asserting both unmatched strings pass through and nil value pointers indicate env var should be omitted from the final interpolated map.
• Integrates interpolated build envs into S2I builder.

/kind enhancement

Fixes #990

[knative-prow]

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[codecov]

Codecov Report

Merging #991 (b4baba7) into main (ce26a23) will increase coverage by 0.08%.
The diff coverage is 63.15%.

@@            Coverage Diff             @@
##             main     #991      +/-   ##
==========================================
+ Coverage   46.05%   46.14%   +0.08%     
==========================================
  Files          57       57              
  Lines        7458     7496      +38     
==========================================
+ Hits         3435     3459      +24     
- Misses       3697     3710      +13     
- Partials      326      327       +1     

Impacted Files	Coverage Δ
s2i/builder.go	26.98% <0.00%> (-5.71%)	⬇️
function.go	60.25% <88.88%> (+3.64%)	⬆️

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[lance]

Nice consolidation - ty
/lgtm