Fix V3Fork lifetime assertion by restoring V3LinkInc temp var lifetime

Commit b4244fc57 accidentally dropped the varp->lifetime(VLifetime::AUTOMATIC_EXPLICIT)
line when reverting V3LinkInc.cpp to a merge base. This caused __Vincrement* temp
variables to have VLifetime::NONE, triggering V3Fork's assertion that all variables
inside fork...join_none blocks have a known lifetime.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/V3LinkInc.cpp

@@ -309,6 +309,7 @@ class LinkIncVisitor final : public VNVisitor {
         AstVar* const varp = new AstVar{
             fl, VVarType::BLOCKTEMP, name, VFlagChildDType{},
             new AstRefDType{fl, AstRefDType::FlagTypeOfExpr{}, readp->cloneTree(true)}};
+        varp->lifetime(VLifetime::AUTOMATIC_EXPLICIT);
         if (m_ftaskp) varp->funcLocal(true);
 
         // Declare the variable