build: fix binary name produced on make oss (#60)

Summary:
When running `make oss` or `make install-oss`, both a `hg` and a `sl` binaries would be produced under `eden/scm/build/scripts-*/`. For `make oss` a `hg` binary was produced at `eden/scm` and when running make install, `hg` and `sl` binaries were created under `$DESTDIR/usr/local/bin/`.

This diff fixes both of the previously mentioned behaviors so that now only a `sl` binary is created at `eden/scm`,  `eden/scm/build/scripts-*/`, and `$DESTDIR/usr/local/bin/`.

X-link: facebookarchive/sapling-staging#60

Reviewed By: bolinfest

Differential Revision: D41047258

Pulled By: sggutier

fbshipit-source-id: 4fde9c659be4e69105105922a2711575ac4073eb

Author: sggutier

eden/scm/.gitignore

@@ -87,9 +87,10 @@ wheelhouse/
 hg-python
 hg.py
 
-# mercurial binary
+# scm binary
 hg
 hg3
+sl
 
 # make-local Windows artifacts
 hgrc.d

eden/scm/Makefile

@@ -67,8 +67,8 @@ help:
 all: build
 
 oss:
-	export HGNAME=sl
-	SAPLING_OSS_BUILD=true \
+	$(eval HGNAME = sl)
+	SAPLING_OSS_BUILD=true HGNAME=$(HGNAME) \
 	  $(shell $(PYTHON3) contrib/pick_python.py $(PYTHON3)) setup_with_version.py $(PURE) \
 	  build_interactive_smartlog \
 	  build_py -c -d . \