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

@@ -78,6 +78,14 @@ verifying checksum
 checksum verified
 ```
 
+## `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