Implement TinyGo Environment Logic and Client Fix

[cdvelop]

This PR implements Stages 4 and 5 of the TinyGo installation plan.

Changes:

• env.go: Added GetRoot() to determine the TinyGo root directory and GetEnv() to generate a modified environment slice containing TINYGOROOT and an updated PATH. This avoids mutating the global process environment.
• env_test.go: Added comprehensive unit tests for both system and local installation scenarios, ensuring correct environment construction and lack of global mutation.
• client/wasmbuild.go: Introduced a new client package that demonstrates the fix for the os.Setenv bug. It uses tinygo.GetEnv() to inject the necessary environment variables directly into the compiler subprocess.

All changes have been verified with go test ./... and go build ./client/....

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PR created automatically by Jules for task 11817058800369912870 started by @cdvelop

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