libstore/build: always treat seccomp setup failures as fatal

In f047e4357b, I missed the behavior that if
building without a dedicated build user (i.e. in single-user setups), seccomp
setup failures are silently ignored. This was introduced without explanation 7
years ago (ff6becafa8). Hopefully the only
use-case nowadays is causing spurious test suite successes when messing up the
seccomp filter during development. Let's try removing it.

Change-Id: Ibe51416d9c7a6dd635c2282990224861adf1ceab

src/libstore/build/local-derivation-goal.cc

@@ -1460,11 +1460,7 @@ void LocalDerivationGoal::runChild()
 
         commonChildInit();
 
-        try {
-            setupSeccomp();
-        } catch (...) {
-            if (buildUser) throw;
-        }
+        setupSeccomp();
 
         bool setUser = true;