The rolling-bootstrap smoke probe ran `$cc --source s.pas --output s` with no
unit-path, relying on the binary finding the RTL beside itself. That worked
while the RTL was an archive (FindRTL looks beside the binary), but once a
commit makes the compiler source-build the RTL, the boot/carried binary is not
beside its compiler/ source tree, so RTLSourceDir (binary-relative, then
CWD-relative) misses and the probe fails with "RTL source directory not found".
Point the probe at the worktree's RTL source via $BLAISE_RTL_SRC and the unit
path. Backward-compatible: pre-source-build commits ignore $BLAISE_RTL_SRC and
still link the archive copied beside the binary; the extra unit paths are
harmless for a program that uses no units.
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