fix: load internal environment variables in Envelope

[eduardoboucas]

Summary

#5338 has changed the way the NETLIFY_DEV environment variable is set. Rather than mutating process.env immediately, we set a variable in the env object with the type internal, which is processed as a type of environment variable that can't be overridden.

Unfortunately, this wasn't added to the Envelope methods, and so the variable was being removed for sites with Envelope enabled.

This PR addresses that and adds some tests (to both legacy and new environment variable systems) to ensure we don't introduce this regression again.

[github-actions]

📊 Benchmark results

Comparing with b0a6c85

Package size: 265 MB

(no change)

^  267 MB  267 MB  267 MB  267 MB  267 MB  267 MB  272 MB  272 MB  272 MB  272 MB                         
│   ┌──┐    ┌──┐    ┌──┐    ┌──┐    ┌──┐    ┌──┐    ┌──┐    ┌──┐    ┌──┐    ┌──┐   265 MB  265 MB  265 MB 
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    ┌──┐    ┌──┐    ┌──┐  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
│   |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |▒▒|  
└───┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴────┴──┴──>
    T-12    T-11    T-10    T-9     T-8     T-7     T-6     T-5     T-4     T-3     T-2     T-1      T    

Legend

• T-25 (bce1f2c): 260 MB
• T-24 (2d3923c): 260 MB
• T-23 (6726bf4): 267 MB
• T-22 (87adddc): 267 MB
• T-21 (6d6531a): 267 MB
• T-20 (f33c595): 267 MB
• T-19 (6594da7): 267 MB
• T-18 (22ecec6): 260 MB
• T-17 (c7c5fff): 267 MB
• T-16 (7e27e7a): 267 MB
• T-15 (89754a1): 267 MB
• T-14 (2aa9341): 267 MB
• T-13 (734c23c): 267 MB
• T-12 (e978613): 267 MB
• T-11 (733c47e): 267 MB
• T-10 (027efcf): 267 MB
• T-9 (99305de): 267 MB
• T-8 (3a9670b): 267 MB
• T-7 (f9ff366): 267 MB
• T-6 (384e168): 272 MB
• T-5 (a0f3719): 272 MB
• T-4 (8146b3d): 272 MB
• T-3 (43e6f9e): 272 MB
• T-2 (4600e9b): 265 MB
• T-1 (b0a6c85): 265 MB
• T (current commit): 265 MB

[jasonbarry]

Thanks @eduardoboucas!! Once all sites are migrated to Envelope in Feb/March, we can delete a lot of code from the CLI that juggles env vars between the two systems. I'm looking forward to it 🔥