fix: partial eval misusing Token.Bytes slice

[i4ki]

The code modified the *hclwrite.Token.Bytes slice directly but it seems that multiple Hashicorp tokens can have its bytes backed by the same byte array, so appending bytes to tok.Bytes was overwriting the byte slice of other tokens.

Closes #376

[codecov]

Codecov Report

Merging #379 (6f4df40) into main (2358a7b) will increase coverage by 0.04%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##             main     #379      +/-   ##
==========================================
+ Coverage   69.30%   69.34%   +0.04%     
==========================================
  Files          37       37              
  Lines        7300     7318      +18     
==========================================
+ Hits         5059     5075      +16     
- Misses       1981     1982       +1     
- Partials      260      261       +1     

Flag	Coverage Δ
tests	69.34% <92.85%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
hcl/eval/partial.go	80.49% <92.85%> (+0.15%)	⬆️

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[katcipis]

LGTM + some small considerations if we should test stack path related things on the genhcl package, but not sure about it, feel free to disregard.