Fix nix develop invocation for verification scripts

[ThVerg]

Fixes #295 together with PR #304 .

Problem

run_script() launches the DRC/LVS scripts through the Nix devShell:

p_cmd = [
    "nix",
    "--extra-experimental-features", "nix-command flakes",
    "develop",
    "--command",
    scriptpath,
]
p = subprocess.Popen(p_cmd, stdout=fo, stderr=fe, cwd=OPTS.openram_temp)

nix develop resolves its flake from the working directory, but the working directory here is OPTS.openram_temp, which contains no flake.nix. The error lands in the script's .err file:

path '/tmp/openram_<user>_<pid>_temp' does not contain a 'flake.nix', searching up
error: could not find a flake.nix file

The script never runs, so no report is written, and verification fails on the missing file rather than on the nix error. That is what #295 reports:

ERROR: file magic.py: line 326: Unable to load LVS results from /tmp/openram_dario_19123_temp/....lvs.report
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/openram_dario_19123_temp/....lvs.report'

Depending on which check runs first the same cause can also surface at the DRC stage as Unable to find the total error line in Magic output.

Change

Name the flake explicitly, so it is resolved from the repository while the script still runs with its working directory in the temp dir — which run_drc.sh relies on for its relative cp of the cell library.

repo_root = os.path.abspath(os.path.join(os.environ["OPENRAM_HOME"], ".."))
p_cmd = ["nix", ..., "develop", "path:{}".format(repo_root), "--command", scriptpath]

OPENRAM_HOME is guaranteed to be set here — __init__.py puts it in os.environ during setup, and install_nix() already derives the flake location the same way.

Verification

sky130 build with use_nix = True (magic 8.3.568, netgen 1.5.323):

• before: could not find a flake.nix file in the .err file, no report produced, run asserts
• after: DRC and LVS both run through the devShell and write their reports

The workaround until this lands is use_nix = False in the config, which uses whatever tools are on PATH.