fix(build): resolve podman at /usr/local/bin

GitHub's ubuntu-24.04 runner image 20260804.265 replaced the apt podman
package with a static bundle installed to /usr/local/bin/podman. The
Justfile only probed /usr/bin/podman, so it silently fell back to docker.

Docker's overlay2 driver caps stacked lower dirs at 128, and
silverblue-main:44 has 259 layers, so every Latest build died with
"failed to register layer: max depth exceeded" before running a single
build script. Podman has no such limit and builds the same base fine.

Probe /usr/local/bin/podman before falling back to docker. Existing
resolution order and the PODMAN env override are unchanged.

Assisted-by: Claude Opus 5 via GitHub Copilot CLI

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: castrojo

Justfile

@@ -16,7 +16,7 @@ tags := '(
     [beta]=beta
 )'
 export SUDOIF := if `id -u` == "0" { "" } else { "sudo" }
-export PODMAN := if path_exists("/usr/bin/podman") == "true" { env("PODMAN", "/usr/bin/podman") } else if path_exists("/usr/bin/docker") == "true" { env("PODMAN", "docker") } else { env("PODMAN", "exit 1 ; ") }
+export PODMAN := if path_exists("/usr/bin/podman") == "true" { env("PODMAN", "/usr/bin/podman") } else if path_exists("/usr/local/bin/podman") == "true" { env("PODMAN", "/usr/local/bin/podman") } else if path_exists("/usr/bin/docker") == "true" { env("PODMAN", "docker") } else { env("PODMAN", "exit 1 ; ") }
 export PULL_POLICY := if PODMAN =~ "docker" { "missing" } else { "newer" }
 just := just_executable()