Check end of path for shell detection

Addresses issue: pypa#6197

Previously these branches would trip if the shell name was present
anywhere in the cmd path. This updates the check to only look at the
end.

news/6197.bugfix.rst

@@ -0,0 +1 @@
+Update shell detection to only check the end of the command used.

pipenv/shells.py

@@ -37,17 +37,17 @@ def _get_activate_script(cmd, venv):
     """
     # Suffix and source command for other shells.
     # Support for fish shell.
-    if "fish" in cmd:
+    if cmd.endswith("fish"):
         suffix = ".fish"
         command = "source"
     # Support for csh shell.
-    elif "csh" in cmd:
+    elif cmd.endswith("csh"):
         suffix = ".csh"
         command = "source"
-    elif "xonsh" in cmd:
+    elif cmd.endswith("xonsh"):
         suffix = ".xsh"
         command = "source"
-    elif "nu" in cmd:
+    elif cmd.endswith("nu"):
         suffix = ".nu"
         command = "overlay use"
     else: