Fix make target hotfix (#11492)

The code inside the `hotfix` taget is overriding the values set at the
beginning of the Makefile affecting other make targets as well.

For example, running `TAG=mytag make docker` also ends up tagging
the docker image as a hotfix instead of `mytag`.

Using the `eval` function inside the `hotfix` target fixes this.

Makefile

@@ -71,10 +71,11 @@ build: checks
 	@echo "Building minio binary to './minio'"
 	@GO111MODULE=on CGO_ENABLED=0 go build -tags kqueue -trimpath --ldflags "$(LDFLAGS)" -o $(PWD)/minio 1>/dev/null
 
-hotfix: LDFLAGS := $(shell MINIO_RELEASE="RELEASE" MINIO_HOTFIX="hotfix.$(shell git rev-parse --short HEAD)" go run buildscripts/gen-ldflags.go $(shell git describe --tags --abbrev=0 | \
-    sed 's#RELEASE\.\([0-9]\+\)-\([0-9]\+\)-\([0-9]\+\)T\([0-9]\+\)-\([0-9]\+\)-\([0-9]\+\)Z#\1-\2-\3T\4:\5:\6Z#'))
-	TAG := "minio/minio:$(shell git describe --tags --abbrev=0).hotfix.$(shell git rev-parse --short HEAD)"
-hotfix: install
+hotfix-vars:
+	$(eval LDFLAGS := $(shell MINIO_RELEASE="RELEASE" MINIO_HOTFIX="hotfix.$(shell git rev-parse --short HEAD)" go run buildscripts/gen-ldflags.go $(shell git describe --tags --abbrev=0 | \
+    sed 's#RELEASE\.\([0-9]\+\)-\([0-9]\+\)-\([0-9]\+\)T\([0-9]\+\)-\([0-9]\+\)-\([0-9]\+\)Z#\1-\2-\3T\4:\5:\6Z#')))
+	$(eval TAG := "minio/minio:$(shell git describe --tags --abbrev=0).hotfix.$(shell git rev-parse --short HEAD)")
+hotfix: hotfix-vars install
 
 docker-hotfix: hotfix checks
 	@echo "Building minio docker image '$(TAG)'"