Since commit 192a9182 (FindLAPACK: MKL clean up and fix for windows,
2013-10-08, v3.0.0-rc1~538^2), FindLAPACK accidentally used FindBLAS's
`BLAS_` prefix for some of its check results.

Since commit 5b8f69eb (FindBLAS: Detect implicitly linked BLAS
library, 2018-08-28, v3.13.0-rc1~150^2~2), FindBLAS stores a check
result in a plain `BLAS_WORKS` variable.  The typo in FindLAPACK happens
to cause a collision with that name.

The typo was already fixed in post-3.13 development as part of other
work in commit 8b63265e (FindLAPACK: Unify internal variables related
to MKL, 2018-11-18).  Fix the typo in the 3.13 version of FindLAPACK to
avoid the collision.  Otherwise it could cause FindLAPACK to incorrectly
determine that a certain library combination does not work (or
incrrectly that it works).

Fixes: #18860