osmo-trx-uhd: Make sure HOME environment variable is set

It turns out that uhd versions >= 4.0.0.0 *require* that either the
HOME or the XDG_CONFIG_HOME variables are set, and otherwise will
terminate the program.

Change-Id: I1816013c507da28719590f063da0a397da656a10
Closes: OS#6269
diff --git a/contrib/systemd/osmo-trx-uhd.service b/contrib/systemd/osmo-trx-uhd.service
index 34ba74a..6c5c2de 100644
--- a/contrib/systemd/osmo-trx-uhd.service
+++ b/contrib/systemd/osmo-trx-uhd.service
@@ -8,6 +8,7 @@
 Restart=always
 StateDirectory=osmocom
 WorkingDirectory=%S/osmocom
+Environment=HOME=%h
 ExecStart=/usr/bin/osmo-trx-uhd -C /etc/osmocom/osmo-trx-uhd.cfg
 RestartSec=2
 # CPU scheduling policy: