Support restoring workspace archives from another OS

[native-api]

trafficstars

by creating any necessary directory hierarchy on the fly

https://travis-ci.community/t/introducing-workspaces/4249/9

This allows to restore absolute paths regardless of the OS.

Tested the command line on a Travis Python archive in Bionic, OSX Catalina and Windows Git Bash.

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The stumbling block is currently that it's impossible to extract stuff to /home in OSX since it's an automounter mount point.

That could be fixed by altering OSX images:

• remove/comment the /home line from /etc/auto_master in OSX images; this must be done in the image, or run sudo launchctl kill HUP system/com.apple.autofsd after changing it.

/home automount point in OSX is only used for mounting a network home dir from a LDAP service, so we don't need it anyway.

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This may still fail in some unusual setups -- e.g. if root is a readonly filesystem (is this the case with Milti-arch VMs?).

[vladimiry]

Any news? Looking forward to https://travis-ci.community/t/allow-to-restore-a-workspace-in-another-os/9165 and https://travis-ci.community/t/workspaces-do-not-work-nicely-with-cross-platform-builds/4461 to be fixed.