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Standardize hubble and cilium CLIs makefile

Most Makefiles in this repository follow a standard pattern, inheriting a lot of their config from the root's Makefile.defs. The cilium-cli and hubble directories Makefiles are notable exceptions. This PR standardizes them to remove duplicated code and logic and to make them behave more like other Makefiles.

Note: this PR is similar with/related to/a followup of #37911

Summary of changes:

cilium-cli/Makefile:

• Remove duplicated variables already sourced from Makefile.defs: GO, GO_BUILD, GO_TAGS
• Remove variable STRIP_DEBUG and DEBUG, the behavior they controlled is now controlled via NOSTRIP
• Use $(GO_TEST) instead of $(GO) test
• Update/Add .DEFAULT_GOAL and .PHONY targets

Impact on make -C cilium-cli:
Before:

make: Entering directory '/home/hadrien/go/src/github.com/DataDog/cilium/cilium-cli'
CGO_ENABLED=0 go build  \
        -ldflags "-w -s -X 'github.com/cilium/cilium/cilium-cli/defaults.CLIVersion=v1.17.0-pre.3-1281-ge8ee0f0770'" \
        -o cilium \
        ./cmd/cilium
make: Leaving directory '/home/hadrien/go/src/github.com/DataDog/cilium/cilium-cli'

After:

make: Entering directory '/home/hadrien/go/src/github.com/DataDog/cilium/cilium-cli'
CGO_ENABLED=0 go build  -mod=vendor -ldflags ' -X "github.com/cilium/cilium/pkg/version.ciliumVersion=1.18.0-dev bbd15b1532 2025-02-18T15:24:14+01:00" -s -w -X "github.com/cilium/cilium/pkg/envoy.requiredEnvoyVersionSHA=c3c35d52ca3b699de1f9448ab7174a9bdcb13f69" -X "github.com/cilium/cilium/cilium-cli/defaults.CLIVersion=v1.17.0-pre.3-1282-gbbd15b1532" ' -tags=osusergo  \
        -o cilium \
        ./cmd/cilium
make: Leaving directory '/home/hadrien/go/src/github.com/DataDog/cilium/cilium-cli'

Difference are the addition of -mod=vendor and -tags=osusergo as well as cilium version variables (currently unused by cilium-cli)

Note: to build the cilium-cli binary with debug symbols unstripped, we now should use NOSTRIP=1 make -C cilium-cli instead of the previous DEBUG=1 make -C cilium-cli

hubble/Makefile:

• Add missing copyright header
• Remove duplicated variables already sourced from Makefile.defs: GO, GO_BUILD_FLAGS, GO_TEST_FLAGS, GO_BUILD, VERSION, GO_TAGS
• Replace hardcoded references to the hubble target with $(TARGET)
• Replace make with $(MAKE)
• Update/Add .DEFAULT_GOAL and .PHONY targets

Impact on make -C hubble:
Before:

make: Entering directory '/home/hadrien/go/src/github.com/DataDog/cilium/hubble'
GOOS= GOARCH= CGO_ENABLED=0 go build  -mod=vendor -ldflags ' -X "github.com/cilium/cilium/pkg/version.ciliumVersion=1.18.0-dev e8ee0f0770 2025-02-28T14:15:28+01:00" -s -w -X "github.com/cilium/cilium/pkg/envoy.requiredEnvoyVersionSHA=c3c35d52ca3b699de1f9448ab7174a9bdcb13f69" ' -tags=osusergo   -ldflags "-w -s -X 'github.com/cilium/cilium/hubble/pkg.GitBranch=main' -X 'github.com/cilium/cilium/hubble/pkg.GitHash=e8ee0f0770' -X 'github.com/cilium/cilium/hubble/pkg.Version=v1.18.0-dev'" -o ./hubble .
make: Leaving directory '/home/hadrien/go/src/github.com/DataDog/cilium/hubble'

After:

make: Entering directory '/home/hadrien/go/src/github.com/DataDog/cilium/hubble'
GOOS= GOARCH=amd64 CGO_ENABLED=0 go build  -mod=vendor -ldflags ' -X "github.com/cilium/cilium/pkg/version.ciliumVersion=1.18.0-dev bbd15b1532 2025-02-18T15:24:14+01:00" -s -w -X "github.com/cilium/cilium/pkg/envoy.requiredEnvoyVersionSHA=c3c35d52ca3b699de1f9448ab7174a9bdcb13f69" -X "github.com/cilium/cilium/hubble/pkg.GitBranch=makefile" -X "github.com/cilium/cilium/hubble/pkg.GitHash=bbd15b1532" -X "github.com/cilium/cilium/hubble/pkg.Version=v1.18.0-dev" ' -tags=osusergo  -o ./hubble .
make: Leaving directory '/home/hadrien/go/src/github.com/DataDog/cilium/hubble'

Difference is that the two separate -ldflags args are now combined as a single one.

operator/Makefile:

• Remove reference to non defined GOCLEAN variable and replace $(GO) clean with $(GO_CLEAN)
• Update/Add .DEFAULT_GOAL and .PHONY targets

Note: the GO_BUILD variable from Makefile.defs already automatically wires up the go build flags and ldflags, simplifying usage.