make: fix gen issue with mockgen not found in PATH (#10162)

Fixes
#10157 (comment)

Problem was:
```
grep -r -l --exclude-dir=erigon-lib "^// Code generated by MockGen. DO NOT EDIT.$" . | xargs rm -r
```

was deleting the `mockgen` binary after it was built 🙃

Makefile

@@ -1,5 +1,6 @@
 GO ?= go # if using docker, should not need to be installed/linked
-GOBIN = $(CURDIR)/build/bin
+GOBINREL = build/bin
+GOBIN = $(CURDIR)/$(GOBINREL)
 UNAME = $(shell uname) # Supported: Darwin, Linux
 DOCKER := $(shell command -v docker 2> /dev/null)
 
@@ -216,7 +217,7 @@ mocks: mocks-clean
 
 ## mocks-clean:                       cleans all generated test mocks
 mocks-clean:
-	grep -r -l --exclude-dir=erigon-lib "^// Code generated by MockGen. DO NOT EDIT.$$" . | xargs rm -r
+	grep -r -l --exclude-dir="erigon-lib" --exclude-dir="*$(GOBINREL)*" "^// Code generated by MockGen. DO NOT EDIT.$$" . | xargs rm -r
 
 ## solc:                              generate all solidity contracts
 solc:

erigon-lib/Makefile

@@ -72,13 +72,13 @@ grpc: protoc-all
 		txpool/txpool.proto txpool/mining.proto
 	rm -rf vendor
 
-$(GOBINREL)/mockgen: | $(GOBINREL)
+build-mockgen:
 	$(GOBUILD) -o "$(GOBIN)/mockgen" go.uber.org/mock/mockgen
 
 mocks-clean:
-	grep -r -l "^// Code generated by MockGen. DO NOT EDIT.$$" . | xargs rm -r
+	grep -r -l --exclude-dir="*$(GOBINREL)*" "^// Code generated by MockGen. DO NOT EDIT.$$" . | xargs rm -r
 
-mocks: $(GOBINREL)/mockgen mocks-clean
+mocks: build-mockgen mocks-clean
 	PATH="$(GOBIN):$(PATH)" go generate -run "mockgen" ./...
 
 lintci-deps: