Move manager binaries under workdir

Brouberolrequested to merge
T364797-workdir into main

We previous;y attempted to force the container to either work with a workdir set to something else than / but with binaries under /, and also to cd / before exec ing the manager binary.

Because none of these attempts fully worked, we're attempting to move all necessary binaries under the default workdir. This way, the operator code that checks for supported architectures, which uses relative paths, should still work, and so should the bootstrap code, that used to fail with

root@deploy1003:/home/brouberol# kubectl logs pgcluster-example-1-initdb-c7wrs -n pgcluster-test -c bootstrap-controller
{"level":"info","ts":"2024-08-05T14:24:39Z","msg":"Installing the manager executable","destination":"/controller/manager","version":"1.23.3","build":{"Version":"1.23.3","Commit":"none","Date":"unknown"}}
panic: open manager: no such file or directory

goroutine 1 [running]:
main.main.NewCmd.func3(0xc00033e308, {0xc00062f9c0?, 0x4?, 0x217d2d8?})
	/build/internal/cmd/manager/bootstrap/cmd.go:44 +0x1d5
github.com/spf13/cobra.(*Command).execute(0xc00033e308, {0xc00062f9a0, 0x2, 0x2})
	/home/somebody/go/pkg/mod/github.com/spf13/cobra@v1.8.1/command.go:985 +0xaca
github.com/spf13/cobra.(*Command).ExecuteC(0xc0001c3b08)
	/home/somebody/go/pkg/mod/github.com/spf13/cobra@v1.8.1/command.go:1117 +0x3ff
github.com/spf13/cobra.(*Command).Execute(...)
	/home/somebody/go/pkg/mod/github.com/spf13/cobra@v1.8.1/command.go:1041
main.main()
	/build/cmd/manager/main.go:66 +0x43e

as it assumed that the manager binary was in the current workdir.

Bug: T364797

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