makefile: set 'HGNAME' env variable when running 'make oss'

Summary:
before

```
$ make oss
...
rm -f sl
cp build/scripts-3*/sl sl
cp: build/scripts-3*/sl: No such file or directory
make: *** [local] Error 1
```
after

```
...
rm -f sl
cp build/scripts-3*/sl sl
```

Reviewed By: muirdm

Differential Revision: D49871832

fbshipit-source-id: ecbd083f5187b6fb0f0c4e640d732f336ef2cb30

eden/scm/Makefile

@@ -80,7 +80,7 @@ install-oss: oss
 	  --install-scripts="$(PREFIX)/bin" --force
 
 local:
-	SAPLING_OSS_BUILD=$(OSS) \
+	SAPLING_OSS_BUILD=$(OSS) HGNAME=$(HGNAME) \
 	  $(shell $(PYTHON3) contrib/pick_python.py $(PYTHON3)) setup.py $(PURE) \
 	  build_interactive_smartlog \
 	  build_py -c -d . \