Baking custom reflection light probes when an .exr file (Texture type: cubemap or HDRI) is assigned as the custom cubemap overwrites the user's file.

Repro steps:

1. Open Test1 scene from attached project.
2. Select Reflection Probe from Hierarchy.
3. Observe the currently selected custom cubemap.
3. Click "Bake".
4. Observe how the cubstom cubemap got overwritten.

Expected outcome: to not overwrite user texture or give a warning that assigned cubemap will be overwritten when an .exr texture file is used.

Notes:
- a cube map of any other texture format than *.exr prompts the user to save new cubemap.
- not a regression, testes as far back as 5.3.1f1.
- Appears to be typical light probe behavior, however since Version 5.5.0a1 (5a6d256ca545) graphics/staging branch more emphasis is added on .exr and .hdr textures, making this potentially a more common issue.