Update Makefile to use CGO_ENABLED environment variable

[sukhil-suresh]

There are scenarios in which CGO_ENABLED=1 has to be set when building the gomplate binary.
Specifically building the binary in FIPS-enabled mode requires that CGO_ENABLED=1 and GOEXPERIMENT=boringcrypto are set.
However, this is not possible with the Makefile hardcoding CGO_ENABLED=0.

This PR makes changes to allow the value of the CGO_ENABLED environment variable to be used if it is set.
If the CGO_ENABLED environment variable is not set, then CGO_ENABLED defaults to 0.

[hairyhenderson]

Thanks for the contribution, @sukhil-suresh!

[hairyhenderson]

LGTM - just need an explanatory comment (I'll definitely forget why we'd ever want to override it!)

[sukhil-suresh]

@hairyhenderson please take another look. I have made the suggested change.

[hairyhenderson]

Thanks @sukhil-suresh!