Can't map Epel-6 (from epel-6) to [... CentOS-6 ...]
Sentry Issue: [PACKIT-SERVICE-3N5](https://sentry.io/organizations/red-hat-0p/issues/2636836262/?referrer=github_integration)

```
Can't map Epel-6 (from epel-6) to ['Fedora-32', 'Fedora-32-aarch64', 'Fedora-33', 'Fedora-33-aarch64', 'Fedora-34', 'Fedora-34-aarch64', 'Fedora-35', 'Fedora-35-aarch64', 'Fedora-Rawhide', 'Fedora-Rawhide-aarch64', 'CentOS-6', 'CentOS-7', 'CentOS-7-aarch64', 'CentOS-8', 'CentOS-8-aarch64', 'CentOS-Stream-8', 'CentOS-Stream-8-aarch64', 'CentOS-Stream-9', 'CentOS-Stream-9-aarch64']
```

Testing Farm [has CentOS-6 compose](https://api.dev.testing-farm.io/v0.1/composes) so this should be just about adding it to [the mapping](https://github.com/packit/packit-service/blob/f7101d20e2e09c4940009225f0cd9ccf644f646f/packit_service/worker/testing_farm.py#L216).

diff --git a/packit_service/worker/testing_farm.py b/packit_service/worker/testing_farm.py
index cf74e091..ac412fe5 100644
--- a/packit_service/worker/testing_farm.py
+++ b/packit_service/worker/testing_farm.py
@@ -207,16 +207,15 @@ class TestingFarmJobHelper(CoprBuildJobHelper):
         """Get distro and arch from chroot."""
         distro, arch = chroot.rsplit("-", 1)
 
+        epel_mapping = {
+            "epel-6": "centos-6",
+            "epel-7": "centos-7",
+        }
+
         if self.job_config.metadata.use_internal_tf:
-            epel_mapping = {
-                "epel-7": "centos-7",
-                "epel-8": "centos-8",
-            }
+            epel_mapping["epel-8"] = "centos-8"
         else:
-            epel_mapping = {
-                "epel-7": "centos-7",
-                "epel-8": "centos-stream-8",
-            }
+            epel_mapping["epel-8"] = "centos-stream-8"
 
         distro = epel_mapping.get(distro, distro)
         return distro, arch
diff --git a/tests/unit/test_testing_farm.py b/tests/unit/test_testing_farm.py
index 3cd223f5..f6b5a700 100644
--- a/tests/unit/test_testing_farm.py
+++ b/tests/unit/test_testing_farm.py
@@ -148,6 +148,7 @@ def test_testing_farm_response(
         ("fedora-33-x86_64", "fedora-33", "x86_64", False),
         ("fedora-rawhide-aarch64", "fedora-rawhide", "aarch64", False),
         ("centos-stream-x86_64", "centos-stream", "x86_64", False),
+        ("epel-6-x86_64", "centos-6", "x86_64", False),
         ("epel-7-x86_64", "centos-7", "x86_64", False),
         ("epel-8-x86_64", "centos-stream-8", "x86_64", False),
         ("fedora-33-x86_64", "fedora-33", "x86_64", True),
['packit_service/worker/testing_farm.py', 'tests/unit/test_testing_farm.py']