[BUG] EXR files are always assumed to be "Linear" without checking a previously existing value for oiio:ColorSpace

When reading an .exr file, the oiio:ColorSpace is always set to "Linear". If the read file already contained this metadata but set to another value (ACEScg for example), it is overwritten. I expected the original value to be retained.

The following piece of C++ code shows how to reproduce the bug:

```
const OIIO::ParamValue* p;

// Read input JPEG image
OIIO::ImageBuf imgBuf("path/to/my/file.jpg");

// Check oiio:ColorSpace value in the input image
p = imgBuf.spec().find_attribute("oiio:ColorSpace");
std::cout << "oiio:ColorSpace = " << ( p ? p->get_string() : "Undefined") << std::endl;

// Color Space conversion from sRGB to ACEScg
OIIO::ColorConfig colorConfig("path/to/my/config.ocio");
OIIO::ImageBuf colorspaceBuf;
OIIO::ImageBufAlgo::colorconvert(colorspaceBuf, imgBuf, "sRGB", "ACEScg", true, "", "", &colorConfig);

// Check oiio:ColorSpace value after conversion
p = colorspaceBuf.spec().find_attribute("oiio:ColorSpace");
std::cout << "oiio:ColorSpace = " << (p ? p->get_string() : "Undefined") << std::endl;

// Save converted image on disk as an .exr file
colorspaceBuf.write("path/to/my/convertedFile.exr");

// Read the stored .exr file
OIIO::ImageBuf imgBufNew("path/to/my/convertedFile.exr");

// Check oiio:ColorSpace value after reading the exr file
p = imgBufNew.spec().find_attribute("oiio:ColorSpace");
std::cout << "oiio:ColorSpace = " << (p ? p->get_string() : "Undefined") << std::endl;
```

This code produces the following output on the console:

```
oiio:ColorSpace = sRGB
oiio:ColorSpace = ACEScg
oiio:ColorSpace = Linear
```

I would expect that the last line to be:

`oiio:ColorSpace = ACEScg`

**Platform information:**
- OIIO picked and compiled through vcpkg package manager.
- OS: Windows 10
- C++ compiler: Visual Studio 2022 / platform x64

The report names OIIO::ImageBuf, ImageBufAlgo::colorconvert, and the EXR read/write path. Start by reproducing the metadata change with the supplied C++ example, then trace how EXR reading handles oiio:ColorSpace. Done means an existing value such as ACEScg is retained after writing and reading the EXR file.

Written by the indexing model from the issue text.