test(e2e): 687 plans in a home with its own configuration

The macOS runner's inherited configuration names its ~/.xlings shim as the
xlings binary, so nothing was vendored and the precondition failed before
either criterion ran. The home now uses its own configuration, whose binary
is the vendored one.

Refs #636

Author: speak-agent

tests/e2e/687_the_vendored_xlings_probe_is_an_argument_vector.sh

@@ -33,7 +33,10 @@ export MCPP_HOME="$TMP/mcpp-home"
 # Offline: configuration loading still acquires the vendored xlings (it copies
 # a local binary), and no bootstrap step reaches the network.
 export MCPP_OFFLINE=1
-source "$(dirname "$0")/_inherit_toolchain.sh"
+# The home's own configuration, whose xlings binary is the vendored one. An
+# inherited configuration may name another (the macOS runner's names its
+# `~/.xlings` shim), and then nothing is vendored and nothing is probed.
+MCPP_INHERIT_CONFIG=0 source "$(dirname "$0")/_inherit_toolchain.sh"
 cd "$TMP"
 
 VENDORED="$MCPP_HOME/registry/bin/xlings$EXE"