fedora: Add skip_if_unavailable=False to Qubes repositories

DNF defaults to skip_if_unavailable=True, so make sure that Qubes
repositories are treated as vital one. Otherwise it would allow an
attacker to cut the user from updates without visible error (when using
PackageKit for example).

Do not set it for unstable repository, as it isn't critical one.

Fixes QubesOS/qubes-issues#1387

misc/qubes-r3.repo

@@ -2,6 +2,7 @@
 name = Qubes OS Repository for VM (updates)
 baseurl = http://yum.qubes-os.org/r3.1/current/vm/fc$releasever
 gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-qubes-3-primary
+skip_if_unavailable=False
 gpgcheck = 1
 enabled=1
 
@@ -9,6 +10,7 @@ enabled=1
 name = Qubes OS Repository for VM (updates-testing)
 baseurl = http://yum.qubes-os.org/r3.1/current-testing/vm/fc$releasever
 gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-qubes-3-primary
+skip_if_unavailable=False
 gpgcheck = 1
 enabled=0
 
@@ -16,6 +18,7 @@ enabled=0
 name = Qubes OS Repository for VM (updates-testing)
 baseurl = http://yum.qubes-os.org/r3.1/security-testing/vm/fc$releasever
 gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-qubes-3-primary
+skip_if_unavailable=False
 gpgcheck = 1
 enabled=0