buildcache push: make --allow-root the default and deprecate the option (#38878)

Without --allow-root spack cannot push binaries that contain paths in
binaries. This flag is almost always needed, so there is no point of
requiring users to spell it out. 

Even without --allow-root, rpaths would still have to be patched, so the 
flag is not there to guarantee binaries are not modified on install.

This commit makes --allow-root the default, and drops the code 
required for it. It also deprecates `spack buildcache preview`, since 
the command made sense only with --allow-root.

As a side effect, Spack no longer depends on binutils for relocation

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