fix: use the "state" directory in systemd unit files

As pointed out by @MrCowKing in, "${HOME}" isn't expanded. We use the
state directory as that's guaranteed to exist (and, in the default
configuration, will match the "HOME" directory of the ipfs user).

See: b47ecd0#commitcomment-83565661

Author: Stebalien

misc/systemd/ipfs-hardened.service

@@ -69,7 +69,7 @@ Type=notify
 User=ipfs
 Group=ipfs
 StateDirectory=ipfs
-Environment=IPFS_PATH="${HOME}"
+Environment=IPFS_PATH="%S"
 ExecStart=/usr/bin/ipfs daemon --init --migrate
 Restart=on-failure
 KillSignal=SIGINT

misc/systemd/ipfs.service

@@ -40,7 +40,7 @@ Type=notify
 User=ipfs
 Group=ipfs
 StateDirectory=ipfs
-Environment=IPFS_PATH="${HOME}"
+Environment=IPFS_PATH="%S"
 ExecStart=/usr/bin/ipfs daemon --init --migrate
 Restart=on-failure
 KillSignal=SIGINT