GLES2 Can't alter projection matrix

[BastiaanOlij]

Godot version:
Godot 3.1 commit 115885b

OS/device including version:
Windows 10, NVidia 1060 GTX

Issue description:
I'm using a trick in my VR version of my racing game tutorial where I alter the project matrix to overrule the Z-output of the projection matrix. This works fine with GLES3 but with GLES2 this gives an error:

ERROR: _display_error_with_code: SceneShaderGLES2: Vertex shader compilation failed:
0(465) : error C7563: assignment to uniform projection_matrix
0(466) : error C7563: assignment to uniform projection_matrix

Steps to reproduce:
Use this as a vertex shader:

void vertex() {
	// adjust our projection to place our resulting z at our near plane
	PROJECTION_MATRIX[2][2] = 0.0;
	PROJECTION_MATRIX[3][2] = -0.1;
}

Then try and use it in GLES2

Minimal reproduction project:
Try using:
https://github.com/BastiaanOlij/vehicle-demo
And switch to GLES2 renderer

[BastiaanOlij]

@reduz any suggestions on fixing this? If we can somehow detect the projection matrix is being assigned we could maybe trigger the use of a copy?