Add env-flutter #104

Job	Run time
windows/env-flutter	48s
linux/env-alpine	29s
linux/env-android:jdk17	1m 52s
linux/env-android:jdk21	2m 8s
linux/env-android:latest	1m 39s
linux/env-archlinux	1m 11s
linux/env-debian	1m 6s
linux/env-fedora	1m 12s
linux/env-flutter	3m 53s
linux/env-opensuse	1m 47s
linux/env-sphinx	1m 9s
linux/env-ubuntu:14.04	1m 29s
linux/env-ubuntu:16.04	1m 52s
linux/env-ubuntu:18.04	1m 36s
linux/env-ubuntu:20.04	3m 8s
linux/env-ubuntu:22.04	4m 12s
linux/env-ubuntu:24.04	4m 16s
linux/env-ubuntu:latest	4m 15s
linux/env-ubuntu:nolibs	51s
linux/toolchain-aarch64-linux-gnu	2m 28s
linux/toolchain-arm-bcm2708hardfp-linux-gnueabi	1m 33s
linux/toolchain-arm-linux-gnueabihf	3m 15s
linux/toolchain-mips-openwrt-linux-atheros	2m 56s
linux/toolchain-linux-android	3m 30s
	52m 35s

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