ci: set buildx 0.8.2 version

[crazy-max]

atm GitHub runner still has buildx 0.8.1 which causes errors in our workflows: https://github.com/moby/buildkit/runs/5845916468?check_suite_focus=true#step:7:728

#122 [linux/s390x buildkit-export 2/3] RUN apk add --no-cache fuse3 git openssh pigz xz   && ln -s fusermount3 /usr/bin/fusermount
#122 CACHED
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xfca460]

goroutine 31 [running]:
github.com/moby/buildkit/util/progress/progressui.(*textMux).print(0xc0008e3e08, 0xc0008e3e80)
	/build/moby-buildx/.gopath/src/github.com/docker/buildx/vendor/github.com/moby/buildkit/util/progress/progressui/printer.go:284 +0x7c0
github.com/moby/buildkit/util/progress/progressui.DisplaySolveStatus(0x255d660, 0xc0002543c0, 0x0, 0x0, 0x0, 0x0, 0x25180c0, 0xc000128010, 0xc00005b380, 0x0, ...)
	/build/moby-buildx/.gopath/src/github.com/docker/buildx/vendor/github.com/moby/buildkit/util/progress/progressui/display.go:83 +0x405
github.com/docker/buildx/util/progress.NewPrinter.func1(0x7fffa321773c, 0x5, 0xc00059dba0, 0x2567118, 0xc000128010, 0x255d660, 0xc0002543c0, 0xc00005b380, 0xc0002cb810, 0xc00005b3e0)
	/build/moby-buildx/.gopath/src/github.com/docker/buildx/util/progress/printer.go:99 +0x132
created by github.com/docker/buildx/util/progress.NewPrinter
	/build/moby-buildx/.gopath/src/github.com/docker/buildx/util/progress/printer.go:87 +0x1e5

set buildx version to 0.8.2 to fix it.

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com