Get rid of Finally in DerivationBuilderImpl::unprepareBuild

Calling `reset` on this `std::optional` field of `DerivationBuilderImpl`
is also what the (automatically created) destructor of
`DerivationBuilderImpl` will do. We should be making sure that the
derivation builder is cleaned up by the goal anyways, and if we do that,
then this `Finally` is no longer needed.

src/libstore/unix/build/derivation-builder.cc

@@ -484,14 +484,6 @@ bool DerivationBuilderImpl::prepareBuild()
 
 SingleDrvOutputs DerivationBuilderImpl::unprepareBuild()
 {
-    // FIXME: get rid of this, rely on RAII.
-    Finally releaseBuildUser([&]() {
-        /* Release the build user at the end of this function. We don't do
-           it right away because we don't want another build grabbing this
-           uid and then messing around with our output. */
-        buildUser.reset();
-    });
-
     /* Since we got an EOF on the logger pipe, the builder is presumed
        to have terminated.  In fact, the builder could also have
        simply have closed its end of the pipe, so just to be sure,