Use [sources] to pin MTK to PR branch for CI testing

Pins ModelingToolkit and ModelingToolkitBase to the as/linearize-op
PR branch (SciML/ModelingToolkit.jl#4443) so CI can validate the
LinearizationOpPoint integration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: baggepinnen

Project.toml

@@ -8,6 +8,7 @@ ControlSystemsBase = "aaaaaaaa-a6ca-5380-bf3e-84a91bcd477e"
 DataInterpolations = "82cc6244-b520-54b8-b5a6-8a565e85f1d0"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 ModelingToolkit = "961ee093-0014-501f-94e3-6117800e7a78"
+ModelingToolkitBase = "7771a370-6774-4173-bd38-47e70ca0b839"
 ModelingToolkitStandardLibrary = "16a59e39-deab-5bd0-87e4-056b12336739"
 MonteCarloMeasurements = "0987c9cc-fe09-11e8-30f0-b96dd679fdca"
 RobustAndOptimalControl = "21fd56a4-db03-40ee-82ee-a87907bee541"
@@ -34,3 +35,7 @@ Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [targets]
 test = ["Test", "ControlSystems", "GenericLinearAlgebra", "OrdinaryDiffEqRosenbrock", "OrdinaryDiffEqNonlinearSolve"]
+
+[sources]
+ModelingToolkit = {url = "https://github.com/SciML/ModelingToolkit.jl", rev = "as/linearize-op"}
+ModelingToolkitBase = {url = "https://github.com/SciML/ModelingToolkit.jl", rev = "as/linearize-op", subdir = "lib/ModelingToolkitBase"}