Rewrite Fedora build version detection

Rewrite so we don't need to constantly update with every new Fedora
release. This is especially useful when Fedora and Jellyfin release
cycles don't line up.

Version selection is as follows:
* TARGET environment variable, which is currently used already
* Currently running Fedora version
* Hardcoded Fallback version that can be updated occasionally

fedora/Makefile

@@ -8,13 +8,17 @@ TARBALL :=$(NAME)-$(subst -,~,$(VERSION)).tar.gz
 
 epel-7-x86_64_repos := https://packages.microsoft.com/rhel/7/prod/
 epel-8-x86_64_repos := https://download.copr.fedorainfracloud.org/results/@dotnet-sig/dotnet-preview/$(TARGET)/
-fedora_repos        := https://download.copr.fedorainfracloud.org/results/@dotnet-sig/dotnet-preview/$(TARGET)/
-fedora-34-x86_64_repos := $(fedora_repos)
-fedora-35-x86_64_repos := $(fedora_repos)
-fedora-34-x86_64_repos := $(fedora_repos)
+
+fed_ver := $(shell rpm -E %fedora)
+# fallback when not running on Fedora
+fed_ver ?= 36
+TARGET  ?= fedora-$(fed_ver)-x86_64
+
+ifeq ($(findstring fedora,$(TARGET)),fedora)
+$(TARGET)_repos     := https://download.copr.fedorainfracloud.org/results/@dotnet-sig/dotnet-preview/$(TARGET)/
+endif
 
 outdir  ?= $(PWD)/$(DIR)/
-TARGET  ?= fedora-35-x86_64
 
 srpm: $(DIR)/$(SRPM)
 tarball: $(DIR)/$(TARBALL)