Widen cargo net retry in required e2e legs

The cargo-vex-matrix lock-v4 leg failed on PR #1449 (run 38307581268,
attempt 1; green on re-run of the same SHA) when a fresh-CARGO_HOME
baseline `cargo build` could not connect to static.crates.io. Cargo
classifies that error as spurious, but its default net.retry of 3
gives up after ~12s, and SOCKET_PATCH_CARGO_E2E_REQUIRED turns the
failed fixture build into a failed leg. In the merge queue the same
blip evicts an entry.

Set CARGO_NET_RETRY=10 (~80s of cargo's own backoff) on the children
of cargo_e2e_matrix::cargo_command when the leg is required and the
caller has not set CARGO_NET_RETRY. Optional local runs keep the
default so an offline machine still skips quickly. Only transfers
cargo itself treats as transient are retried; checksum and resolver
failures still fail at once.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GTcGpX2RVXH9BXATkDJLdm

Author: claude

crates/socket-patch-cli/tests/cargo_e2e_matrix/mod.rs

@@ -80,9 +80,20 @@ pub fn skip(suite: &str, why: &str) -> bool {
     true
 }
 
+/// `net.retry` for the children of a [`required`] leg. Cargo retries only
+/// the transfers it classifies as spurious (connect failures, timeouts,
+/// 5xx) and backs off ~1s, 3.5s, 6.5s, 9.5s, then 10s per attempt, so 10
+/// rides out a ~80s crates.io / runner network blip where the default 3
+/// gave up after ~11s and failed the leg (a fresh-CARGO_HOME baseline build
+/// hit "Could not connect to static.crates.io" in the lock-v4 leg).
+const REQUIRED_NET_RETRY: &str = "10";
+
 /// A `cargo` child with the fixture's private `CARGO_HOME`, the pinned
 /// toolchain, and no ambient `CARGO_TARGET_DIR` (shared-build-cache setups
-/// would redirect child builds out of the fixture).
+/// would redirect child builds out of the fixture). On a [`required`] leg
+/// it also widens cargo's own network retry (an explicit ambient
+/// `CARGO_NET_RETRY` wins); an optional run keeps the default so an
+/// offline machine still skips quickly.
 pub fn cargo_command(cwd: &Path, cargo_home: &Path) -> Command {
     let mut cmd = Command::new("cargo");
     cmd.current_dir(cwd)
@@ -91,6 +102,9 @@ pub fn cargo_command(cwd: &Path, cargo_home: &Path) -> Command {
     if let Some(tc) = toolchain() {
         cmd.env("RUSTUP_TOOLCHAIN", tc);
     }
+    if required() && env_nonempty("CARGO_NET_RETRY").is_none() {
+        cmd.env("CARGO_NET_RETRY", REQUIRED_NET_RETRY);
+    }
     cmd
 }