Use correct `target_os` for DragonFlyBSD #172

Job	Run time
Rustfmt	6s
Test (ubuntu-latest, stable)	14s
Test (ubuntu-latest, beta)	18s
Test (ubuntu-latest, nightly)	18s
Test (macos-latest, stable)	24s
Test (macos-latest, beta)	24s
Test (macos-latest, nightly)	24s
Test (windows-latest, stable)	2m 31s
Test (windows-latest, beta)	1m 28s
Test (windows-latest, nightly)	1m 4s
Test Cross Compile - aarch64-apple-ios	19s
Test Cross Compile - aarch64-unknown-linux-gnu	48s
Test Cross Compile - arm-unknown-linux-gnueabihf	42s
Test Cross Compile - mips-unknown-linux-gnu	6s
Test Cross Compile - mips64-unknown-linux-gnuabi64	6s
Test Cross Compile - mips64el-unknown-linux-gnuabi64	3s
Test Cross Compile - mipsel-unknown-linux-gnu	4s
Test Cross Compile - powerpc-unknown-linux-gnu	29s
Test Cross Compile - s390x-unknown-linux-gnu	32s
Test Cross Compile - x86_64-unknown-linux-musl	38s
Test Cross Compile - aarch64-unknown-linux-musl	40s
Test Cross Compile - x86_64-pc-windows-gnu	58s
Test Cross Compile - x86_64-unknown-freebsd	30s
Test Cross Compile - x86_64-unknown-netbsd	40s
Test Cross Compile - x86_64-sun-solaris	5s
Test Cross Compile - x86_64-unknown-illumos	43s
	14m 34s

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