exec-env: don't touch GO{ROOT,PATH} if ASDF_GOLANG_DISABLE_GO{ROOT,PATH} is 1

Setting these variables is not really required for things to work, and
some setups explicitly prefer them not to be touched.

Modeled after the similar options in goenv.
https://github.com/syndbg/goenv/blob/d4e2dd79b83496dbf9474e31dbdb8d7eb8bb0261/ENVIRONMENT_VARIABLES.md

Author: scop

README.md

@@ -56,8 +56,8 @@ selected, not necessarily `1.14.patch`.
 
 ## Architecture Override
 
-The `ASDF_GOLANG_OVERWRITE_ARCH` variable can be used to override the architecture 
-that is used for determining which Go build to download. The primary use case is when attempting 
+The `ASDF_GOLANG_OVERWRITE_ARCH` variable can be used to override the architecture
+that is used for determining which Go build to download. The primary use case is when attempting
 to install an older version of Go for use on an Apple M1 computer as Go was not being built for ARM at the time.
 
 #### Without ASDF_GOLANG_OVERWRITE_ARCH
@@ -86,6 +86,14 @@ checksum verified
 By default we try to verify the checksum of each install but ocassionally [that's not possible](https://github.com/kennyp/asdf-golang/issues/91).
 If you need to skip the checksum for some reason just set `ASDF_GOLANG_SKIP_CHECKSUM`.
 
+## `GOROOT` and `GOPATH` Overrides
+
+By default, `$GOROOT` and `$GOPATH` are set to the selected version's
+`.../go` and `.../packages` dirs, and unset if the selected version is
+`system`. Set the `$ASDF_GOLANG_DISABLE_GOROOT` and/or
+`$ASDF_GOLANG_DISABLE_GOPATH` environment variables to `1` to disable
+this behavior for the respective variables.
+
 ## Contributing
 
 Feel free to create an issue or pull request if you find a bug.

bin/exec-env

@@ -1,8 +1,11 @@
 #!/usr/bin/env bash
 
 if [ "${ASDF_INSTALL_VERSION}" = 'system' ] ; then
-	unset -v GOROOT GOPATH
+	[ "${ASDF_GOLANG_DISABLE_GOROOT-}" = '1' ] || unset -v GOROOT
+	[ "${ASDF_GOLANG_DISABLE_GOPATH-}" = '1' ] || unset -v GOPATH
 else
-	export GOROOT=$ASDF_INSTALL_PATH/go
-	export GOPATH=$ASDF_INSTALL_PATH/packages
+	[ "${ASDF_GOLANG_DISABLE_GOROOT-}" = '1' ] ||
+	    export GOROOT=$ASDF_INSTALL_PATH/go
+	[ "${ASDF_GOLANG_DISABLE_GOPATH-}" = '1' ] ||
+	    export GOPATH=$ASDF_INSTALL_PATH/packages
 fi