Release shared lock before acquiring exclusive lock

In principle, this should avoid deadlocks where two instances of Nix are
holding a shared lock on big-lock and are both waiting to get an
exclusive lock.

However, it seems like `flock(2)` is supposed to do this automatically,
so it's not clear whether this is actually where the problem comes from.

src/libstore/local-store.cc

@@ -91,6 +91,7 @@ void migrateCASchema(SQLite& db, Path schemaPath, AutoCloseFD& lockFd)
 
         if (!lockFile(lockFd.get(), ltWrite, false)) {
             printInfo("waiting for exclusive access to the Nix store for ca drvs...");
+            lockFile(lockFd.get(), ltNone, false); // We have acquired a shared lock; release it to prevent deadlocks
             lockFile(lockFd.get(), ltWrite, true);
         }
 
@@ -299,6 +300,7 @@ LocalStore::LocalStore(const Params & params)
 
         if (!lockFile(globalLock.get(), ltWrite, false)) {
             printInfo("waiting for exclusive access to the Nix store...");
+            lockFile(globalLock.get(), ltNone, false); // We have acquired a shared lock; release it to prevent deadlocks
             lockFile(globalLock.get(), ltWrite, true);
         }