Revert "freedesktop-sdk.bst: update to 21.08 beta2"

This reverts commit 4c50aa38.

This seems to have broken 'meson scan-build' again. Epiphany CI is failing with:

$ flatpak build flatpak_app ninja -C _build scan-build
ninja: Entering directory `_build'
ninja: error: unknown target 'scan-build'