Fixes make clean on MacOS

[rdoxenham]

This patch fixes a very simple issue when building the elemental3 and elemental3ctl binaries on macOS, where calling make clean results in an error:

% make clean
find: -executable: unknown primary or operator
make: *** [clean] Error 1

The problem is that the Makefile expects to be using a Linux-based find utility, where the -executable switch works just fine, but this is not available on macOS. Here's the original line:

@find . -type f -executable -name '*.test' -exec rm -f {} \+

A simple fix here is to introduce a new check as part of the find call, where it actually uses test to see if each file it encounters is executable, and if so, then pass it over to rm -f. This bypasses the OS-specific parameter entirely.

Testing of this patch, firstly on Linux:

% grep -i pretty /etc/os-release
PRETTY_NAME="openSUSE Tumbleweed"

% uname -r
7.0.11-1-default

% make
go build -ldflags '-w -s -X "github.com/suse/elemental/v3/internal/cli/cmd.version=v3.1.0-alpha.20260528" -X "github.com/suse/elemental/v3/internal/cli/cmd.gitCommit=be4a555ace67e1e30967b10f0331d902bc6fd496"' -o build/elemental3 ./cmd/elemental
go build -ldflags '-w -s -X "github.com/suse/elemental/v3/internal/cli/cmd.version=v3.1.0-alpha.20260528" -X "github.com/suse/elemental/v3/internal/cli/cmd.gitCommit=be4a555ace67e1e30967b10f0331d902bc6fd496"' -o build/elemental3ctl ./cmd/elemental3ctl

% make clean
removed './build/elemental3'
removed './build/elemental3ctl'
removed directory './build'

Then on macOS:

% uname -v
Darwin Kernel Version 25.5.0: Mon Apr 27 20:39:09 PDT 2026; root:xnu-12377.121.6~2/RELEASE_ARM64_T6020

% make
go build -ldflags '-w -s -X "github.com/suse/elemental/v3/internal/cli/cmd.version=v0.0.1" -X "github.com/suse/elemental/v3/internal/cli/cmd.gitCommit=17228df94e58c6a7f13daf10deb5146b98aba7a0"' -o build/elemental3 ./cmd/elemental
go build -ldflags '-w -s -X "github.com/suse/elemental/v3/internal/cli/cmd.version=v0.0.1" -X "github.com/suse/elemental/v3/internal/cli/cmd.gitCommit=17228df94e58c6a7f13daf10deb5146b98aba7a0"' -o build/elemental3ctl ./cmd/elemental3ctl

% make clean
./build/elemental3ctl
./build/elemental3
./build

Fixes: #493

[dirkmueller]

I think practically you could also do -perm /100 instead of -executable, avoids the -exec test call. yes, tehre is a small difference (one just checks unix permissions, the other one also ACLs) but that's not something we actually have.