dashboard: set GO{,HOST}ARCH in linux-386-longtest builder

The linux-386-longtest builder was added in CL 202479 with these
environment variables unset, which caused the builder to run long
tests for the linux/amd64 pair. Setting these to "386" makes the
builder run long tests for linux/386 instead, which increases
32-bit test coverage as originally intended.

For golang/go#29641.

Change-Id: I9fbebc9e37e5f27f21d840d86006192daeaee8c5
Reviewed-on: https://go-review.googlesource.com/c/build/+/234520
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>

dashboard/builders.go

@@ -1779,6 +1779,8 @@ func init() {
 		},
 		needsGoProxy: true, // for cmd/go module tests
 		env: []string{
+			"GOARCH=386",
+			"GOHOSTARCH=386",
 			"GO_TEST_TIMEOUT_SCALE=5", // give them lots of time
 		},
 	})