Diagnostic: force sccache recache for modules build

Reintroduces test_on.cpp into the modular build (reverting the
CMakeLists.txt change from 8f6735f) and sets SCCACHE_RECACHE=1 for the
modules matrix entry.

Goal: isolate whether the CI ICEs on this matrix entry (blocking merges
on test_on.cpp in particular) are caused by a stale/corrupted sccache
cache entry rather than a live, per-run compiler issue. This changes
exactly one variable relative to the last known-failing config
(e39bbcf, serialized -j1, test_on.cpp included, cache active): caching
behavior for that one TU.

If this passes: cache is implicated. Follow up by removing
SCCACHE_RECACHE and rerunning to confirm the now-fresh entry is read
back successfully, before deciding whether to leave RECACHE on
permanently for this matrix entry.

If this still ICEs: cache is ruled out as the (sole) cause; the CI
failure is likely a genuine, deterministic issue in the CI Clang build
(22.1.8, x86_64 Ubuntu) independent of caching.

Author: ispeters

.github/workflows/ci.cpu.yml

@@ -78,6 +78,10 @@ jobs:
           SCCACHE_DIST_URL: "https://amd64.linux.sccache.rapids.nvidia.com"
           SCCACHE_IDLE_TIMEOUT: "0"
           SCCACHE_NO_DIST_COMPILE: ${{ contains(matrix.name, 'modules') && '1' || '' }}
+          # Diagnostic: force sccache to ignore/overwrite any existing cache entries for
+          # the modules build, to rule out a stale/corrupted cached artifact as the cause
+          # of the ICEs we've been seeing on this matrix entry (e.g. test_on.cpp).
+          SCCACHE_RECACHE: ${{ contains(matrix.name, 'modules') && '1' || '' }}
           SCCACHE_REGION: "us-east-2"
           SCCACHE_S3_KEY_PREFIX: "nvidia-stdexec-dev"
           SCCACHE_S3_PREPROCESSOR_CACHE_KEY_PREFIX: "nvidia-stdexec-dev/preprocessor"

test/CMakeLists.txt

@@ -32,6 +32,7 @@ set(stdexec_test_sources
     stdexec/algos/adaptors/test_finally.cpp
     stdexec/algos/adaptors/test_into_variant.cpp
     stdexec/algos/adaptors/test_let_error.cpp
+    stdexec/algos/adaptors/test_on.cpp
     stdexec/algos/adaptors/test_on3.cpp
     stdexec/algos/adaptors/test_sequence.cpp
     stdexec/algos/adaptors/test_starts_on.cpp
@@ -81,7 +82,6 @@ if(NOT STDEXEC_BUILD_MODULES)
     stdexec/algos/adaptors/test_bulk.cpp
     stdexec/algos/adaptors/test_let_stopped.cpp
     stdexec/algos/adaptors/test_let_value.cpp
-    stdexec/algos/adaptors/test_on.cpp
     stdexec/algos/adaptors/test_on2.cpp
     stdexec/algos/adaptors/test_spawn_future.cpp
     stdexec/algos/adaptors/test_stopped_as_error.cpp