When we build a copr image, we run the "nm-copr-build.sh" script.
That script, should honor "LTO=0|1|" to explicitly enable/disable
LTO. Since the copr script only builds a SRPM, which then gets build
we need that the default LTO flag in the SRPM is templated.
Fixes: 0566e9dc63 ('contrib: support disabling "LTO" in "nm-copr-build.sh"')
The "nm-copr-build.sh" script is run by our copr to generate the SRPM of
NetworkManager (via `curl ... | bash`).
Building with LTO takes a long time, for testing it can be nice to disable
that. Add an environment variable for that. It can be used when manually
building an RPM in copr.
