build: fix the make build if the GOPATH env not set

Use `go env` to retrieve the `GOPATH` instead of a regular
system environment.

Author: lopezator

Makefile

@@ -23,7 +23,7 @@ build:
 			EXT=".exe"; \
 		fi; \
 		docker run -it --rm \
-		-v $(GOPATH)/src/github.com/lopezator/sqlfmt:/go/src/github.com/lopezator/sqlfmt \
+		-v $(shell go env GOPATH)/src/github.com/lopezator/sqlfmt:/go/src/github.com/lopezator/sqlfmt \
 		-w /go/src/github.com/lopezator/sqlfmt \
 		-e CGO_ENABLED=1 \
 		$(BUILDER) \
@@ -41,4 +41,4 @@ sql-fmt:
 sql-check:
 	@echo "Running sql-check..."
 	@find . -name '*.sql' -print0 | xargs -I '{}' -0 \
-	bash -c 'diff {} <(sqlfmt < {})' > /dev/null || (echo "sql-check failed, run \"make sql-fmt\" and try again"; exit 1)
+	bash -c 'diff {} <(sqlfmt < {})' > /dev/null || (echo "sql-check failed, run \"make sql-fmt\" and try again"; exit 1)