Fix using platform with install_command

Fixes tox-dev#1464

Author: jayvdb

docs/changelog/1464.bugfix.rst

@@ -0,0 +1 @@
+Fix ``platform`` support for ``install_command``. - by :user:`jayvdb`

src/tox/config/__init__.py

@@ -1672,13 +1672,24 @@ def getargv(self, name, default="", replace=True):
         return self.getargvlist(name, default, replace=replace)[0]
 
     def getargv_install_command(self, name, default="", replace=True):
+        def _unescape_install_command_args(s):
+            s = s.replace(r"\{packages\}", "{packages}")
+            s = s.replace(r"\{opts\}", "{opts}")
+            return s
+
         s = self.getstring(name, default, replace=False)
         if "{packages}" in s:
             s = s.replace("{packages}", r"\{packages\}")
         if "{opts}" in s:
             s = s.replace("{opts}", r"\{opts\}")
 
-        return _ArgvlistReader.getargvlist(self, s, replace=replace)[0]
+        values = _ArgvlistReader.getargvlist(self, s, replace=replace)
+        if not values:
+            # This occurs when factors are used, and a testenv doesnt have
+            # a factorised value for install_command, most commonly occurring
+            # if setting platform is also used.
+            return _unescape_install_command_args(default)
+        return values[0]
 
     def getstring(self, name, default=None, replace=True, crossonly=False, no_fallback=False):
         x = None