chore(internal/gapicgen): use -d to download modules (#3744)

Related golang/go#33526
diff --git a/internal/gapicgen/cmd/genbot/README.md b/internal/gapicgen/cmd/genbot/README.md
index b601ca8..08268e7 100644
--- a/internal/gapicgen/cmd/genbot/README.md
+++ b/internal/gapicgen/cmd/genbot/README.md
@@ -109,7 +109,7 @@
 
 ```shell
 cd /path/to/internal/gapicgen
-go get -u github.com/googleapis/gapic-generator-go/cmd/protoc-gen-go_gapic
+go get -d github.com/googleapis/gapic-generator-go/cmd/protoc-gen-go_gapic
 ```
 
 (it's just based on the go.mod entry)
diff --git a/internal/gapicgen/cmd/genbot/update.go b/internal/gapicgen/cmd/genbot/update.go
index b11a183..49e8cf6 100644
--- a/internal/gapicgen/cmd/genbot/update.go
+++ b/internal/gapicgen/cmd/genbot/update.go
@@ -70,8 +70,8 @@
 		# Update genproto and api to latest for every module (latest version is
 		# always correct version). tidy will remove the dependencies if they're not
 		# actually used by the client.
-		go get -u google.golang.org/api | true # We don't care that there's no files at root.
-		go get -u google.golang.org/genproto | true # We don't care that there's no files at root.
+		go get -d google.golang.org/api | true # We don't care that there's no files at root.
+		go get -d google.golang.org/genproto | true # We don't care that there's no files at root.
 		go mod tidy;
 	popd;
 done