Revert "Disable tests against Optim temporarily" (#409)

Reverts #407

I just came across #407. It seems that Optim CI was updated and fixed
recently.

Author: devmotion

.github/workflows/downstream.yml

@@ -47,7 +47,7 @@ jobs:
           - {repo: BandedMatrices.jl, group: JuliaLinearAlgebra}
           - {repo: BlockBandedMatrices.jl, group: JuliaLinearAlgebra}
           - {repo: InfiniteLinearAlgebra.jl, group: JuliaLinearAlgebra}
-          # - {repo: Optim.jl, group: JuliaNLSolvers} # Enable when their CI is fixed
+          - {repo: Optim.jl, group: JuliaNLSolvers}
 
     steps:
       - uses: actions/checkout@v4