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

The `.variables` sets `CGO_ENABLED=1` on arm; https://github.com/docker/cli/blob/b0c41b78d8c80c5f8faa736cf3c74dc31092e443/scripts/build/.variables#L57-L68
And if enabled, it sets `-buildmode=pie`; https://github.com/docker/cli/blob/b0c41b78d8c80c5f8faa736cf3c74dc31092e443/scripts/build/.variables#L79-L88

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 0.110 Building static docker-helloworld
    #18 0.110 + GO111MODULE=auto
    #18 0.110 + go build -o /out/plugins-linux-arm/docker-helloworld -tags '' -ldflags ' -X "github.com/docker/cli/cli/version.GitCommit=050d9d6" -X "github.com/docker/cli/cli/version.BuildTime=2024-09-03T09:19:05Z" -X "github.com/docker/cli/cli/version.Version=pr-5387"' github.com/docker/cli/cli-plugins/examples/helloworld
    ....

Signed-off-by: Sebastiaan van Stijn <[email protected]>

scripts/build/plugins

@@ -5,6 +5,12 @@
 
 set -eu -o pipefail
 
+# Disable CGO - we don't need it for these plugins.
+#
+# Important: this must be done before sourcing "./scripts/build/.variables",
+# because some other variables are conditionally set whether CGO is enabled.
+export CGO_ENABLED=0
+
 source ./scripts/build/.variables
 
 for p in cli-plugins/examples/* "$@" ; do
@@ -15,5 +21,5 @@ for p in cli-plugins/examples/* "$@" ; do
     mkdir -p "$(dirname "${TARGET_PLUGIN}")"
 
     echo "Building $GO_LINKMODE $(basename "${TARGET_PLUGIN}")"
-    (set -x ; CGO_ENABLED=0 GO111MODULE=auto go build -o "${TARGET_PLUGIN}" -tags "${GO_BUILDTAGS}" -ldflags "${GO_LDFLAGS}" ${GO_BUILDMODE} "github.com/docker/cli/${p}")
+    (set -x ; GO111MODULE=auto go build -o "${TARGET_PLUGIN}" -tags "${GO_BUILDTAGS}" -ldflags "${GO_LDFLAGS}" ${GO_BUILDMODE} "github.com/docker/cli/${p}")
 done