[CELL-297] CI thin builds no longer OOM-killed on 8 GB ARM runners — nix max-jobs capped at 2 via DEVCELL_NIX_MAX_JOBS

- fix(runner): make nix `max-jobs` configurable via `DEVCELL_NIX_MAX_JOBS` env var (default `auto`) — CI can cap concurrent nix builds to stay within runner memory limits without affecting local dev
- fix(runner): forward `DEVCELL_NIX_MAX_JOBS` into the thin-build docker container via `-e` — env var now reaches the nix.conf inside the builder
- fix(ci): set `DEVCELL_NIX_MAX_JOBS=2` on both thin build steps — ultimate stack no longer exhausts 8 GB Blacksmith runner memory during home-manager switch
- test(runner): update `TestThinBuildArgv_NixConfDaemonMode` to match new `DEVCELL_NIX_MAX_JOBS:-auto` pattern — no behavior change

Author: DimmKirr

.github/workflows/build.dev.yml

@@ -164,6 +164,7 @@ jobs:
         env:
           DEVCELL_NIX_VOLUME: devcell-nix-store-${{ matrix.arch }}
           DEVCELL_NIXHOME_PATH: ${{ github.workspace }}/nixhome
+          DEVCELL_NIX_MAX_JOBS: "2"
         run: |
           BASE_TAG="${{ env.REGISTRY }}/${{ env.IMAGE_NAME_LC }}:v0.0.0-${{ matrix.arch }}-base"
           ./bin/cell build --thin --stack base --image "$BASE_TAG" --debug
@@ -188,6 +189,7 @@ jobs:
         env:
           DEVCELL_NIX_VOLUME: devcell-nix-store-${{ matrix.arch }}
           DEVCELL_NIXHOME_PATH: ${{ github.workspace }}/nixhome
+          DEVCELL_NIX_MAX_JOBS: "2"
         run: |
           ULT_TAG="${{ env.REGISTRY }}/${{ env.IMAGE_NAME_LC }}:v0.0.0-${{ matrix.arch }}-ultimate"
           ./bin/cell build --thin --stack ultimate --image "$ULT_TAG" --debug

internal/runner/thin_build.go

@@ -106,7 +106,7 @@ experimental-features = nix-command flakes
 # 16 is the upstream default; stays under the CDN's throttle threshold.
 max-substitution-jobs = 16
 http-connections = 16
-max-jobs = auto
+max-jobs = ''${DEVCELL_NIX_MAX_JOBS:-auto}
 sandbox = true
 ssl-cert-file = $CACERT
 NIXCONF
@@ -351,6 +351,9 @@ echo "Done — thin image: %s"`,
 	if !remote {
 		args = append(args, "-v", nixhomeRef+":/opt/nixhome")
 	}
+	if v := os.Getenv("DEVCELL_NIX_MAX_JOBS"); v != "" {
+		args = append(args, "-e", "DEVCELL_NIX_MAX_JOBS="+v)
+	}
 	args = append(args,
 		"-v", "/var/run/docker.sock:/var/run/docker.sock",
 		"--entrypoint", "sh",

internal/runner/thin_build_test.go

@@ -137,8 +137,8 @@ func TestThinBuildArgv_NixConfDaemonMode(t *testing.T) {
 	if !strings.Contains(script, "sandbox = true") {
 		t.Error("should set sandbox = true (isolate builds)")
 	}
-	if !strings.Contains(script, "max-jobs = auto") {
-		t.Error("should set max-jobs = auto (parallel builds under daemon)")
+	if !strings.Contains(script, "DEVCELL_NIX_MAX_JOBS:-auto") {
+		t.Error("should set max-jobs from DEVCELL_NIX_MAX_JOBS (default auto)")
 	}
 	if !strings.Contains(script, "nix-daemon") {
 		t.Error("should start nix-daemon (avoids /homeless-shelter race)")