mayaUsdExport wrongly translates file texture names with envvar expansions.

[jufrantz]

Describe the bug

We use environment variables expansion in file texture paths for asset management needs. This is supported by maya file node but not by maya-usd export, which results in invalid usd asset paths.

Steps to reproduce

import os
from maya import cmds
from pxr import Usd, UsdShade
cmds.loadPlugin("mayaUsdPlugin", quiet=True)

1. Prepare environment and open scene from mayaUSDExport_file_envvar_expansion.zip

MY_TEST_DIR = "/path/to/issue_files/mayaUSDExport_file_envvar_expansion"
os.environ["MY_TEST_DIR"] = MY_TEST_DIR
cmds.file(os.path.join(MY_TEST_DIR, "scene.ma"), open=True, force=True)

2. It reads a texture file with MY_TEST_DIR expansion, this is supported by file node

maya.cmds.getAttr("file1.ftn")
# Result: $MY_TEST_DIR/images/grid.png

3. Export it

outStage = Usd.Stage.CreateInMemory()
cmds.mayaUSDExport(file=outStage.GetRootLayer().identifier)

4. Notice that the file path was wrongly translated

UsdShade.Shader.Get(outStage, "/pPlane1/mtl/lambert2SG/file1").GetInput("file").Get()
# Result: Sdf.AssetPath('/path/to/issue_files/mayaUSDExport_file_envvar_expansion/$MY_TEST_DIR/images/grid.png')

Expected behavior

mayaUSDExport should produce an asset path that is valid for USD's default resolver.

Specs (if applicable):

• CentOS 7.8
• Maya 2023.3
• maya-usd 0.27.0
• USD 23.11