fix(ci): conleche-nightly cache path is lambda-only — mkdir denied on clean-room

[noahgift]

conleche-nightly has never gone green since #4655. CI_CACHE=/mnt/nvme-raid0/ci-cache/pvl-conleche exists only on lambda. On the clean-room runner (intel), /mnt/nvme-raid0 is root-owned, so the elan step fails with mkdir: Permission denied (runs 37082537768, 37163229207).

The fix computes the cache dir beside the runner's _work and exports it through GITHUB_ENV. PVL_CONLECHE_CACHE="$CI_CACHE/pins" is unchanged. No check changes.

Refs #4681 (B2: no check red on main >24 h)

🤖 Generated with Claude Code

[noahgift]

quorum-review (AD-04): NOT agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-4715",
 "head": "232691cb53a6d544953d805c10af667c8c51e896",
 "width": 3,
 "executor": "agy",
 "agreed": false,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "NO-VERDICT",
   "findings": 0
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[noahgift]

quorum-review (AD-04): three PASS — agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-4715",
 "head": "232691cb53a6d544953d805c10af667c8c51e896",
 "width": 3,
 "executor": "agy",
 "agreed": true,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[github-actions]

§13.11 rung 1 — quorum shadow verdict

S13-SHADOW pr=4708 head=232691cb53a6d544953d805c10af667c8c51e896 verdict=REFUSE class=Q1 arm_rc=1

Shadow mode: this records a verdict and merges nothing. A refusal
to arm is not a block (§13 adds zero rows to §7) — the pull request is
exactly as green as it was.

[noahgift]

Folded into #4777 (one PR for the five B2 nightly fixes). The fix commit is moved there unchanged; this branch is kept.