cmd/buildlet: use same VERSION for all builds in 'make compile' or 'make upload'

As written before, the shell command was re-executed for each
make recipe, producing a different date for each build. This is closer
to what upload was doing before, but not as useful as everything
having the same version for the same make invocation. Do that instead.

Change-Id: I3b4366060d88c90ea9626458de63e25a4ab1f8cc
Reviewed-on: https://go-review.googlesource.com/c/build/+/420799
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: rsc

cmd/buildlet/Makefile

@@ -75,9 +75,9 @@ clean:
 # Compile everything without uploading.
 compile: $(ALL)
 
-GOFILES=$(shell ls *.go)
+GOFILES:=$(shell ls *.go)
 
-VERSION=$(USER)-$(shell date -u +'%Y-%m-%dT%H:%M:%SZ')
+VERSION:=$(USER)-$(shell date -u +'%Y-%m-%dT%H:%M:%SZ')
 
 BUILD=CGO_ENABLED=0 $(GO) build -ldflags='-X main.Version=$(VERSION)' -o