templater: add joinEnv helper for PATH-like vars

[SergioChan]

Summary

• add a new joinEnv template helper that joins path-like values with the OS-specific path list separator (: on POSIX, ; on Windows)
• expose joinEnv in the templater function map so Taskfiles can build cross-platform PATH values cleanly
• add unit coverage for both direct helper behavior and template function registration

Testing

• git diff --check
• go test ./internal/templater (not runnable in this environment: required Go toolchain go1.25 is unavailable via GOTOOLCHAIN download)

Related

Fixes #2406