cli: dont check dirname presence in action_build

Copr build <projectname:dirname> should according to documentation
create the dirname if does not exists, but the early check check_before_build
in action_build actually runs /build/check-before-build endpoint even
before the <projectname:dirname> has a chance to be created, thus
complains about non-existent dirname.
Passing only projectname to the check should be enough for the check as
is since the /build/check-before-build is called once more after dirname
creation.

Fixes fedora-copr#2786

cli/copr_cli/main.py

@@ -325,7 +325,14 @@ def action_build(self, args):
         # Before we start uploading potentially large source RPM file, make sure
         # that the user has valid credentials and can build in the project.
         self.client.build_proxy.check_before_build(
-            username, projectname, project_dirname, buildopts)
+            ownername=username,
+            projectname=projectname,
+            # documentation says user can create directory with copr build, but
+            # /build/check-before-build endpoint checks for presence of this
+            # dirname even before creating it in this check.
+            project_dirname=None,
+            buildopts=buildopts,
+        )
 
         builds = []
         for pkg in args.pkgs:

python/copr/v3/proxies/build.py

@@ -162,6 +162,9 @@ def check_before_build(self, ownername, projectname,
         :param buildopts: http://python-copr.readthedocs.io/en/latest/client_v3/build_options.html
         :return: Munch
         """
+        if project_dirname is None:
+            project_dirname = projectname
+
         endpoint = "/build/check-before-build"
         data = {
             "ownername": ownername,