fix(build): instruct users to use GOTAGS, not GOFLAGS

GOFLAGS will _override_ all the flags. If we just want to set a tag e.g., enable
OpenSSL, we should set GOTAGS=openssl.

fixes ipfs#6622 (comment)

CHANGELOG.md

@@ -425,7 +425,7 @@ receiving multiple inbound connections per second.
 To enable openssl support, rebuild go-ipfs with:
 
 ```bash
-> make build GOFLAGS=-tags=openssl
+> make build GOTAGS=openssl
 ```
 
 ### CoreAPI