correct strat mixing type toggleability check in discordBDStratTNT #477

Job	Run time
Set-Matrix-Private	1s
Set-Matrix-Private	1s
windows-latest (r release, flags binaries)	1m 2s
windows-latest (r release, flags binaries)	1m 9s
macOS-latest (r release, flags binaries, ubsan)	1m 1s
macOS-latest (r release, flags binaries, ubsan)	1m 1s
ubuntu-latest (r release, flags full, ubsan, debug)	53s
ubuntu-latest (r release, flags full, ubsan, debug)	56s
ubuntu-latest (r devel, flags vignettes, remote)	1m 15s
ubuntu-latest (r devel, flags vignettes, remote)	1m 17s
ubuntu-latest (r release, flags full, covr)	59s
ubuntu-latest (r release, flags full, covr)	1m 15s
	10m 50s

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