cmd/go/internal/cache: use == to test for io.EOF

The documentation of io.EOF: Read must return EOF itself, not an error
wrapping EOF, because callers will test for EOF using ==.

encoding/json package provides an example "ExampleDecoder" which uses
"err == io.EOF" as well, so I think it's more idiomatic to use == to test for io.EOF.

Change-Id: I8a9f06d655ca63b3ec3e7dbbdfc519a2686980e1
GitHub-Last-Rev: 665929e2a2
GitHub-Pull-Request: golang/go#62012
Reviewed-on: https://go-review.googlesource.com/c/go/+/519156
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/cmd/go/internal/cache/prog.go

@@ -229,7 +229,7 @@ func (c *ProgCache) readLoop(readLoopDone chan<- struct{}) {
 			if c.closing.Load() {
 				return // quietly
 			}
-			if errors.Is(err, io.EOF) {
+			if err == io.EOF {
 				c.mu.Lock()
 				inFlight := len(c.inFlight)
 				c.mu.Unlock()