[20.10] use GO_LDFLAGS instead of LDFLAGS to prevent inheriting unrelated options

When building on Fedora 36, the build failed. I suspect this is because the
rpm tools also set LDFLAGS, but with options that cannot be used;

    GO_LINKMODE=dynamic
    + ./scripts/build/binary
      /go/src/github.com/docker/cli ~/rpmbuild/BUILD/src
      Building dynamic docker-linux-arm64
      + go build -o build/docker-linux-arm64 -tags ' pkcs11' -ldflags '-Wl,-z,relro -Wl,--as-needed  -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1  -Wl,--build-id=sha1 -Wl,-dT,/root/rpmbuild/BUILD/src/.package_note-docker-ce-cli-0.0.0.20220330082637.68cad50-0.fc36.aarch64.ld -w -X "github.com/docker/cli/cli/version.GitCommit=68cad50" -X "github.com/docker/cli/cli/version.BuildTime=2022-03-30T20:05:36Z" -X "github.com/docker/cli/cli/version.Version=0.0.0-20220330082637-68cad50" -X "github.com/docker/cli/cli/version.PlatformName=Docker Engine - Community"' -buildmode=pie github.com/docker/cli/cmd/docker
    # github.com/docker/cli/cmd/docker
    flag provided but not defined: -Wl,-z,relro
    usage: link [options] main.o

This patch changes the variable we use to `GO_LDFLAGS`, taking a similar approach
as containerd, and various other projects using this name: https://grep.app/search?q=GO_LDFLAGS

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 391e6ad944)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

scripts/build/.variables

@@ -13,13 +13,13 @@ if test -n "${PLATFORM}"; then
 	PLATFORM_LDFLAGS="-X \"github.com/docker/cli/cli/version.PlatformName=${PLATFORM}\""
 fi
 
-export LDFLAGS="\
+export GO_LDFLAGS="\
     -w \
     ${PLATFORM_LDFLAGS} \
     -X \"github.com/docker/cli/cli/version.GitCommit=${GITCOMMIT}\" \
     -X \"github.com/docker/cli/cli/version.BuildTime=${BUILDTIME}\" \
     -X \"github.com/docker/cli/cli/version.Version=${VERSION}\" \
-    ${LDFLAGS:-} \
+    ${GO_LDFLAGS:-} \
 "
 
 GOOS="$(go env GOOS)"