Use egg in pip url

Otherwise, for some reason, the conda environment created by asv installs the released version of pymbolic.

asv.conf.json

@@ -71,6 +71,7 @@
         "sympy" : ["1.1.1"],
         "pip+git+https://github.com/inducer/pymbolic#egg=pymbolic": [""],
         "pip+git+https://github.com/inducer/islpy#egg=islpy": [""],
+        "pybind11" : [""],
         "pip+git+https://github.com/inducer/pyopencl#egg=pyopencl": [""],
         "pip+git+https://gitlab.tiker.net/inducer/boxtree#egg=boxtree": [""],
         "pip+git+https://github.com/inducer/loopy#egg=loopy": [""],