fix(attest): require absolute env path overrides

[kvinwang]

Summary

Follow-up to #782 for the Copilot review that landed after the PR was merged:

• require non-empty absolute paths for DCAP_TDX_QUOTE_CONFIGFS_PATH, DCAP_TDX_RTMR_SYSFS_PATH, and DSTACK_CCEL_FILE
• invalid configfs overrides do not make is_tdx_available() return true via a relative cwd path
• RTMR overrides can no longer silently write digests under the process working directory

Test plan

• cargo test -p tdx-attest -p cc-eventlog

[Copilot]

Pull request overview

Tightens attestation-related environment path overrides to require non-empty absolute paths, preventing relative values from accidentally resolving against the process working directory (notably for TDX ConfigFS detection and RTMR sysfs extension paths, and CCEL file decoding).

Changes:

• Added a shared helper to validate environment path overrides as non-empty and absolute, and applied it to DCAP_TDX_QUOTE_CONFIGFS_PATH and DCAP_TDX_RTMR_SYSFS_PATH.
• Made is_tdx_available() ignore invalid (e.g., relative) ConfigFS overrides instead of potentially validating against the current working directory.
• Added DSTACK_CCEL_FILE resolution with explicit rejection of empty/relative overrides, plus targeted unit tests.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dstack/tdx-attest/src/linux.rs	Adds absolute-path validation for ConfigFS/RTMR env overrides; prevents relative overrides from influencing TDX availability and RTMR behavior; adds tests.
dstack/cc-eventlog/src/tcg.rs	Introduces ccel_file_path() to enforce absolute CCEL override paths and adds tests for override/default behavior.

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