Release notes: https://github.com/r-lib/processx/releases/tag/v3.9.0

The previous version (3.8.6) failed to build against math/R 4.6.0.
Rf_findVar is a non-API entry point whose declaration is no longer
available in R 4.6.0's installed headers. The vendored copy of
cleancall in processx called Rf_findVar directly in both cleancall.c and
init.c, causing a compile error. 3.9.0 fixes this upstream by replacing
the calls with R_getVar, with a compatibility shim for R < 4.5.0.