[CI:BUILD] rpm: Add fallback for $SOURCE_DATE_EPOCH not being set

At least in all recent Fedora releases, SOURCE_DATE_EPOCH would be set. But the
build should work even if it not set, e.g. in copr builds on c8s. Add 'date +%s'
as a fallback.

From: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>

This is a copy-paste of commit eaa62a52 in podman fedora dist-git.

[NO NEW TESTS NEEDED]

Signed-off-by: Lokesh Mandvekar <lsm5@redhat.com>

rpm/podman.spec

@@ -219,7 +219,7 @@ export CGO_CFLAGS+=" -m64 -mtune=generic -fcf-protection=full"
 
 export GOPROXY=direct
 
-LDFLAGS="-X %{ld_libpod}/define.buildInfo=${SOURCE_DATE_EPOCH:?} \
+LDFLAGS="-X %{ld_libpod}/define.buildInfo=${SOURCE_DATE_EPOCH:-$(date +%s)} \
          -X %{ld_libpod}/config._installPrefix=%{_prefix} \
          -X %{ld_libpod}/config._etcDir=%{_sysconfdir} \
          -X %{ld_project}/pkg/systemd/quadlet._binDir=%{_bindir}"