Fix test-ribasim-core by adding using Pkg; (#742)

Otherwise you would receive an error during `pixi run test-ribasim-core`

pixi.toml

@@ -73,7 +73,7 @@ build = { depends_on = ["build-ribasim-cli", "build-libribasim"] }
 test-ribasim-python = "pytest --numprocesses=auto python/ribasim/tests"
 test-ribasim-api = "pytest --basetemp=python/ribasim_api/tests/temp --junitxml=report.xml python/ribasim_api/tests"
 test-ribasim-cli = "pytest --numprocesses=auto --basetemp=build/ribasim_cli/tests/temp --junitxml=report.xml build/ribasim_cli/tests"
-test-ribasim-core = { cmd = "julia --project=core --eval 'Pkg.test()'", depends_on = [
+test-ribasim-core = { cmd = "julia --project=core --eval 'using Pkg; Pkg.test()'", depends_on = [
     "instantiate-julia-core",
     "generate-testmodels",
 ] }