CI: suppress postgres vaccuuming.

```
[gw1] [ 98%] PASSED tests/test_wallet.py::test_hsmtool_dump_descriptors 
tests/test_wallet.py::test_fundchannel_listtransaction 
[gw0] [ 98%] PASSED tests/test_plugin.py::test_channel_opened_notification 
tests/test_wallet.py::test_hsmtool_generatehsm 
[gw0] [ 98%] PASSED tests/test_wallet.py::test_hsmtool_generatehsm 
tests/test_wallet.py::test_withdraw_nlocktime_fuzz 
[gw1] [ 98%] ERROR tests/test_wallet.py::test_fundchannel_listtransaction 
tests/test_wallet.py::test_fundchannel_listtransaction 
tests/test_wallet.py::test_withdraw_nlocktime_fuzz 
tests/test_wallet.py::test_fundchannel_listtransaction 
[gw0] [ 99%] ERROR tests/test_wallet.py::test_withdraw_nlocktime_fuzz 
tests/test_wallet.py::test_multiwithdraw_simple 
[gw1] [ 99%] ERROR tests/test_wallet.py::test_fundchannel_listtransaction 
tests/test_wallet.py::test_withdraw_nlocktime 
tests/test_wallet.py::test_multiwithdraw_simple 
tests/test_wallet.py::test_withdraw_nlocktime 
tests/test_wallet.py::test_multiwithdraw_simple 
tests/test_wallet.py::test_withdraw_nlocktime 
[gw0] [ 99%] ERROR tests/test_wallet.py::test_multiwithdraw_simple 
tests/test_wallet.py::test_repro_4258 
[gw1] [ 99%] ERROR tests/test_wallet.py::test_withdraw_nlocktime
...
2021-10-12 06:36:09.203 UTC [224552] STATEMENT:  SELECT version FROM version LIMIT 1
2021-10-12 06:36:09.566 UTC [224523] PANIC:  could not write to file "pg_wal/xlogtemp.224523": No space left on device
2021-10-12 06:36:09.566 UTC [224523] STATEMENT:  VACUUM FULL;
Error vacuuming db: BEGIN command failed: PANIC:  could not write to file "pg_wal/xlogtemp.224523": No space left on device
server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
```

Author: rustyrussell

.github/scripts/build.sh

@@ -19,6 +19,7 @@ export TEST_NETWORK=${NETWORK:-"regtest"}
 export TIMEOUT=900
 export VALGRIND=${VALGRIND:-0}
 export FUZZING=${FUZZING:-0}
+export LIGHTNINGD_POSTGRES_NO_VACUUM=1
 
 pip3 install --user -U \
      -r requirements.lock

wallet/db_postgres.c

@@ -254,6 +254,14 @@ static void db_postgres_teardown(struct db *db)
 static bool db_postgres_vacuum(struct db *db)
 {
 	PGresult *res;
+
+#if DEVELOPER
+	/* This can use a lot of diskspacem breaking CI! */
+	if (getenv("LIGHTNINGD_POSTGRES_NO_VACUUM")
+	    && streq(getenv("LIGHTNINGD_POSTGRES_NO_VACUUM"), "1"))
+		return true;
+#endif
+
 	res = PQexec(db->conn, "VACUUM FULL;");
 	if (PQresultStatus(res) != PGRES_COMMAND_OK) {
 		db->error = tal_fmt(db, "BEGIN command failed: %s",