lnd postStart: make more idiomatic

- [[]]-style tests
- indent all multi-line statements the same way

modules/lnd.nix

@@ -100,8 +100,7 @@ in {
 
         sleep 5
 
-        if [ ! -f /secrets/lnd-seed-mnemonic ]
-        then
+        if [[ ! -f /secrets/lnd-seed-mnemonic ]]; then
           echo Creating lnd seed
 
           ${pkgs.curl}/bin/curl -s \
@@ -109,8 +108,7 @@ in {
             -X GET https://127.0.0.1:8080/v1/genseed | ${pkgs.jq}/bin/jq -c '.cipher_seed_mnemonic' > /secrets/lnd-seed-mnemonic
         fi
 
-        if [ ! -f ${cfg.dataDir}/chain/bitcoin/mainnet/wallet.db ]
-        then
+        if [[ ! -f ${cfg.dataDir}/chain/bitcoin/mainnet/wallet.db ]]; then
           echo Creating lnd wallet
 
           ${pkgs.curl}/bin/curl -s \