muster release v5.31.9

self-update replaces the binary with a single rename. The verified download is renamed over the running muster, symbolic links resolved (selfupdatecosign.Install, selfupdate-cosign v0.3.0). go-selfupdate’s own swap moved the binary aside before it moved the new one in, so a muster started in between found none, and concurrent updates could lose it. Now a muster started meanwhile runs the old binary or the new one, several updates may run at once, and the binary keeps its mode.