scripts/build/plugins: don't override CGO_ENABLED set by .variables

[thaJeztah]

• relates to gha: add linux/arm/v5 to list of platforms #5388
• relates to update to go1.22.6 #5387

The .variables sets CGO_ENABLED=1 on arm;

cli/scripts/build/.variables

Lines 57 to 68 in b0c41b7

	if [ -z "$CGO_ENABLED" ]; then
	case "$(go env GOOS)" in
	linux)
	case "$(go env GOARCH)" in
	amd64|arm64|arm|s390x|riscv64)
	CGO_ENABLED=1
	;;
	*)
	CGO_ENABLED=0
	;;
	esac
	;;

And if enabled, it sets -buildmode=pie;

cli/scripts/build/.variables

Lines 79 to 88 in b0c41b7

	if [ "$CGO_ENABLED" = "1" ] && [ "$(go env GOOS)" != "windows" ]; then
	case "$(go env GOARCH)" in
	mips*|ppc64)
	# pie build mode is not supported on mips architectures
	;;
	*)
	GO_BUILDMODE="-buildmode=pie"
	;;
	esac
	fi

But that looks to be conflicting with the hardcoded CGO_ENABLED=0 in this script, which causes the build to fail on go1.22;

> [build-plugins 1/1] RUN --mount=ro --mount=type=cache,target=/root/.cache     xx-go --wrap &&     TARGET=/out ./scripts/build/plugins e2e/cli-plugins/plugins/*:
0.127 Building static docker-helloworld
0.127 + CGO_ENABLED=0
0.127 + GO111MODULE=auto
0.127 + go build -o /out/plugins-linux-arm/docker-helloworld -tags ' osusergo' -ldflags ' -X "github.com/docker/cli/cli/version.GitCommit=5c123b1" -X "github.com/docker/cli/cli/version.BuildTime=2024-09-02T13:52:17Z" -X "github.com/docker/cli/cli/version.Version=pr-5387" -extldflags -static' -buildmode=pie github.com/docker/cli/cli-plugins/examples/helloworld
0.135 -buildmode=pie requires external (cgo) linking, but cgo is not enabled

This patch sets the CGO_ENABLED variable before sourcing .variables, so that other variables which are conditionally set are handled correctly.

Before this PR:

#18 [build-plugins 1/1] RUN --mount=ro --mount=type=cache,target=/root/.cache     xx-go --wrap &&     TARGET=/out ./scripts/build/plugins e2e/cli-plugins/plugins/*
#18 0.123 Building static docker-helloworld
#18 0.124 + CGO_ENABLED=0
#18 0.124 + GO111MODULE=auto
#18 0.124 + go build -o /out/plugins-linux-arm/docker-helloworld -tags ' osusergo' -ldflags ' -X "github.com/docker/cli/cli/version.GitCommit=c8c402e" -X "github.com/docker/cli/cli/version.BuildTime=2024-09-03T08:28:25Z" -X "github.com/docker/cli/cli/version.Version=pr-5381" -extldflags -static' -buildmode=pie github.com/docker/cli/cli-plugins/examples/helloworld
....

With this PR:

#18 [build-plugins 1/1] RUN --mount=ro --mount=type=cache,target=/root/.cache     xx-go --wrap &&     TARGET=/out ./scripts/build/plugins e2e/cli-plugins/plugins/*
#18 0.123 Building static docker-helloworld
#18 0.124 + GO111MODULE=auto
#18 0.124 + go build -o /out/plugins-linux-arm/docker-helloworld -tags ' osusergo' -ldflags ' -X "github.com/docker/cli/cli/version.GitCommit=ecb650e" -X "github.com/docker/cli/cli/version.BuildTime=2024-09-03T08:56:01Z" -X "github.com/docker/cli/cli/version.Version=pr-5387" -extldflags -static' -buildmode=pie github.com/docker/cli/cli-plugins/examples/helloworld
....

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.94%. Comparing base (092b5f0) to head (9e29967).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5393   +/-   ##
=======================================
  Coverage   60.94%   60.94%           
=======================================
  Files         304      304           
  Lines       21346    21346           
=======================================
  Hits        13010    13010           
  Misses       7407     7407           
  Partials      929      929