fix detect_pax behavior on linux if procfs is mounted with hidepid>=1

Merged Bitbucket Importer requested to merge bitbucket/merged-pr-581 into branch/default

Created originally on Bitbucket by tpruzina (Tomáš Pružina)

Was already merged in Bitbucket before import, marked as merged by the import user

PID1 (init) isn't observable on systems with procfs mounted with hidepid=1,2 unless build runs under root (for example on Gentoo where package manager compiles under user 'portage'). This can be fixed by replacing /proc/1/status with /proc/self/status (which is visible to the build script).

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