Use pip install --force-reinstall flag

Under some conditions, pip appears to wrongly think the packages is
available in the environment, so tell it to install it always.

Closes #805

asv/environment.py

@@ -660,8 +660,10 @@ def _install_project(self, repo, commit_hash, build_dir):
         """
         cmd = self._install_command
         if cmd is None:
-            # Run pip via python -m pip, avoids shebang length limit on Linux
-            cmd = ["python -mpip install {wheel_file}"]
+            # Run pip via python -m pip, avoids shebang length limit on Linux.
+            # Ensure --force-reinstall so that package being present e.g. on cwd
+            # does not mess things up.
+            cmd = ["python -mpip install --force-reinstall {wheel_file}"]
 
         if cmd:
             commit_name = repo.get_decorated_hash(commit_hash, 8)

docs/source/asv.conf.json.rst

@@ -66,7 +66,7 @@ Airspeed Velocity rebuilds the project as needed, using these commands.
 The defaults are::
 
   "install_command":
-  ["python -mpip install {wheel_file}"],
+  ["python -mpip install --force-reinstall {wheel_file}"],
 
   "uninstall_command":
   ["return-code=any python -mpip uninstall -y {project}"],