bake: basic variable validation #4757

Job	Run time
prepare	5s
validate (lint, darwin/amd64)	2m 4s
validate (lint, darwin/arm64)	1m 56s
validate (lint, freebsd/amd64)	1m 56s
validate (lint, freebsd/arm64)	1m 52s
validate (lint, linux/amd64)	1m 56s
validate (lint, linux/arm64)	1m 58s
validate (lint, linux/s390x)	2m 2s
validate (lint, linux/ppc64le)	1m 54s
validate (lint, linux/riscv64)	1m 58s
validate (lint, windows/amd64)	2m 3s
validate (lint, windows/arm64)	1m 52s
validate (lint-gopls, darwin/amd64)	3m 46s
validate (lint-gopls, darwin/arm64)	3m 43s
validate (lint-gopls, freebsd/amd64)	3m 57s
validate (lint-gopls, freebsd/arm64)	3m 59s
validate (lint-gopls, linux/amd64)	3m 58s
validate (lint-gopls, linux/arm64)	4m 1s
validate (lint-gopls, linux/s390x)	4m 0s
validate (lint-gopls, linux/ppc64le)	3m 57s
validate (lint-gopls, linux/riscv64)	3m 58s
validate (lint-gopls, windows/amd64)	4m 2s
validate (lint-gopls, windows/arm64)	3m 56s
validate (validate-docs)	1m 21s
validate (validate-golangci)	25s
validate (validate-vendor)	46s
	1h 7m 25s

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