Fix stage0_bin failing in GH Action SLSA generator

Change-Id: I0db40f70db1117188246f9f3fc1210b79f2aa68b

Author: Juliette Pretot

buildconfigs/stage0_bin.toml

@@ -1,18 +1,5 @@
 # This is the static build configuration that we use with the docker-based SLSA3 generator for
 # building the `stage0` binary, and its provenance.
 # See https://github.com/slsa-framework/slsa-github-generator/tree/main/internal/builders/docker.
-command = [
-  "nix",
-  "develop",
-  ".#rust",
-  "--command",
-  "env",
-  "--chdir=stage0_bin",
-  "cargo",
-  "objcopy",
-  "--release",
-  "--",
-  "--output-target=binary",
-  "target/x86_64-unknown-none/release/stage0_bin",
-]
+command = ["nix", "develop", ".#rust", "--command", "just", "stage0_bin"]
 artifact_path = "./stage0_bin/target/x86_64-unknown-none/release/stage0_bin"