CI: test building Oscar manual (#2689)

It is big and may help detect regressions early.

Author: fingolfin

.github/workflows/regression-tests.yml

@@ -62,6 +62,7 @@ jobs:
       matrix:
         include:
           - package: 'MathOptInterface'
+          - package: 'Oscar'
     steps:
       - uses: actions/checkout@v4
       - uses: julia-actions/setup-julia@v2