chore: add local binaries to PATH during installation

[sergeytimoshin]

On fresh installs, download-gnark-keys.sh fails with "go: command not found" (exit code 127) because go is installed to .local/go/bin/ but PATH isn't updated until devenv.sh is sourced after installation.
Add GOROOT and PATH exports to shared.sh so locally installed binaries (go, cargo) are available to subsequent installation steps.

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch sergey/fix-go-path

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