MainCamera's targetTexture is set to a RenderTexture, which has FilterMode.Point. But when the scene is played, filterMode changes to FilterMode.Bilinear.

To reproduce:
1. Open the attached project
2. Open "test" scene
3. Play the scene

Desired outcome: The cube in the scene has "pixel art" look, with sharp edges
Actual result: The cube in the scene is blurry

Reproduced with 5.6.0p4, 5.6.1f1, 5.6.1p1, 2017.2.0a1
Not reproduced with 5.5.2p2, 5.5.3p4, 5.6.0p3, 2017.1.0a4, 2017.1.0b7
Regression introduced in 5.6.0p4

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It is caused by an improvement that was made to allow MSAA if not rendering direct to the back buffer. In this case what is happening is that the project has MSAA settings, and the camera is wanting to use MSAA. As the temporary texture does not have MSAA unity creates an intermediate target. When the MSAA resolve happens the samples are resolved in a way that leads to the blurriness you are seeing.

If you disable 'Allow MSAA' and 'Allow HDR' then the camera will just use the render target you have plugged in.