Revert "Restore parent mount namespace before executing a child process"

This reverts commit a0ef21262f. This
doesn't work in 'nix run' and nix-shell because setns() fails in
multithreaded programs, and Boehm GC mark threads are uncancellable.

Fixes #2646.

src/nix-build/nix-build.cc

@@ -401,6 +401,8 @@ static void _main(int argc, char * * argv)
             } else
                 env[var.first] = var.second;
 
+        restoreAffinity();
+
         /* Run a shell using the derivation's environment.  For
            convenience, source $stdenv/setup to setup additional
            environment variables and shell functions.  Also don't
@@ -444,9 +446,7 @@ static void _main(int argc, char * * argv)
 
         auto argPtrs = stringsToCharPtrs(args);
 
-        restoreAffinity();
         restoreSignals();
-        restoreMountNamespace();
 
         execvp(shell.c_str(), argPtrs.data());