feat: add netrcFile and goModFile parameters for private modules and IFD avoidance

[aaomidi]

Adds support for authenticated private module fetches via netrc:

• New netrcFile parameter in buildGoApplication and fetchGoModule
• Reads netrc file at eval time, passes content to fetch.sh
• fetch.sh writes netrc to $HOME/.netrc for go mod download

Adds goModFile parameter to avoid IFD when pwd is a derivation:

• When pwd points to a derivation output, reading go.mod forces IFD
• goModFile allows passing a raw path that's copied at eval time
• Removes pathExists check which also caused IFD issues

Users can pass GOPROXY as an impure env var to use a custom proxy.