infra(argo): disable CARGO_INCREMENTAL in ensure-binary for sccache hit rate

The project's Cargo.toml and .cargo/config.toml both set
`incremental = true`, which makes rustc emit per-query save-analysis
artifacts that sccache does not cache. Result: even though
RUSTC_WRAPPER=sccache was set, the rustc calls wrote incremental
state that subsequent builds saw as stale and recompiled anyway.

Setting CARGO_INCREMENTAL=0 in the ensure-binary env forces rustc
to emit pure object output that sccache hashes uniformly on
(source + args). Same SHA → full cache hit; small source change →
only dirty crates recompile.

Does not change the service compile-and-deploy-template path,
which intentionally uses cargo-incremental against a persistent
/cargo-target-cpu PVC for repeated rebuilds of a narrow set of
service binaries.

Applies from the next workflow submission onward (train-5wb4n
already compiled under the old config and is running).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

infra/k8s/argo/train-template.yaml

@@ -166,12 +166,21 @@ spec:
           # pods and commits (cargo's incremental cache is invalidated by
           # git checkout mtime touches — sccache is content-hash keyed so it
           # isn't). Binary assumed present in ci-builder image.
+          #
+          # CARGO_INCREMENTAL=0 is required to make sccache effective:
+          # the project's Cargo.toml / .cargo/config.toml set
+          # `incremental = true`, which emits per-query save-analysis
+          # artifacts that sccache does not cache. Turning incremental off
+          # lets rustc emit pure object output that sccache hashes
+          # uniformly — same source + same args → cache hit.
           - name: RUSTC_WRAPPER
             value: sccache
           - name: SCCACHE_DIR
             value: /cargo-target/sccache
           - name: SCCACHE_CACHE_SIZE
             value: "40G"
+          - name: CARGO_INCREMENTAL
+            value: "0"
           - name: GITLAB_PAT
             valueFrom:
               secretKeyRef: