Do not lowercase CHIP_MODULE_CACHE_DIR

[pvelesko]

Fixes #1396

CHIP_MODULE_CACHE_DIR was read through readEnvVar with Lower = true, which lowercases the whole string. The value is a filesystem path, so any uppercase letter in it sent the cache somewhere else, or, when the lowercased path was not creatable, aborted the process with a filesystem_error before anything ran.

The other path-valued variables already pass false, so this just makes CHIP_MODULE_CACHE_DIR consistent with CHIP_JIT_FLAGS, CHIP_JIT_FLAGS_OVERRIDE and CHIP_DUMP_PROCESSED_SPIRV.

Before and after on Arc B570, LLVM 22.0-native, Level Zero and OpenCL, running the new regression test:

# before
requested '/tmp/TestFix1396ChipStarCacheDir' exists=0, lowercased '/tmp/testfix1396chipstarcachedir' exists=1
FAIL

# after
requested '/tmp/TestFix1396ChipStarCacheDir' exists=1, lowercased '/tmp/testfix1396chipstarcachedir' exists=0
PASS

The silent case is the expensive one. On Aurora, pointing the variable at a project directory meant nothing was ever cached, so every process recompiled the whole device module: single gtest cases of kynema-ugf took about 300 seconds each, and a 131 suite campaign got through one suite in a one hour job.

The test re-executes itself rather than calling setenv and carrying on, because chipStar reads its configuration when the driver initializes, which happens before main. The parent sets the variable, the child compiles a kernel, and the parent inspects the directories once the child has exited.

[colleeneb]

/run-aurora-ci

[colleeneb]

/run-aurora-ci

[colleeneb]

/run-aurora-ci