build: use sed to remove v2 path from btcec/coverage.txt

The `goveralls` tool we use to handle code coverage upload seems to not
understand that a `v2 module can exist, without having a v2 file path on
disk. We use a `sed` command to remove the `v2` module prefix so the
tool can reach into the correct file to extract the source code.

Makefile

@@ -87,8 +87,13 @@ unit:
 unit-cover: $(GOACC_BIN)
 	@$(call print, "Running unit coverage tests.")
 	$(GOACC_BIN) ./...
-	cd btcec; $(GOACC_BIN) ./...
+
+	# We need to remove the /v2 pathing from the module to have it work
+	# nicely with the CI tool we use to render live code coverage.
+	cd btcec; $(GOACC_BIN) ./...; sed -i.bak 's/v2\///g' coverage.txt
+
 	cd btcutil; $(GOACC_BIN) ./...
+
 	cd btcutil/psbt; $(GOACC_BIN) ./...
 
 unit-race: