- fix bashism

- py-cryptography cannot usually be built from source without patches;
it relies on a rust crate with a restrictive version check. allow using
it from packages instead.

Index: test/Makefile.in
--- test/Makefile.in.orig
+++ test/Makefile.in
@@ -5,16 +5,16 @@ SHELL=bash
 all: test
 
 test: venv/bin/pytest fakekey
-	(source ./venv/bin/activate; pytest --hostkey=fakekey --dbclient=../dbclient --dropbear=../dropbear $(srcdir) )
+	(. ./venv/bin/activate; pytest --hostkey=fakekey --dbclient=../dbclient --dropbear=../dropbear $(srcdir) )
 
 one: venv/bin/pytest fakekey
-	(source ./venv/bin/activate; pytest --hostkey=fakekey --dbclient=../dbclient --dropbear=../dropbear $(srcdir) -k exit)
+	(. ./venv/bin/activate; pytest --hostkey=fakekey --dbclient=../dbclient --dropbear=../dropbear $(srcdir) -k exit)
 
 fakekey:
 	../dropbearkey -t ecdsa -f $@
 
 venv/bin/pytest: $(srcdir)/requirements.txt
-	python3 -m venv init venv
+	python3 -m venv init venv --system-site-packages
 	./venv/bin/pip install --upgrade pip
 	./venv/bin/pip install -r $(srcdir)/requirements.txt
 
