<keyring> is not readable by user '_apt' executing apt-key

[TJ]

On 18.04 amd64 whilst trying to build an 18.04 package:

W: copy:///<<BUILDDIR>>/resolver-uIDqUN/apt_archive/./Release.gpg: The key(s) in the keyring /etc/apt/trusted.gpg.d/sbuild-build-depends-archive.gpg are ignored as the file is not readable by user '_apt' executing apt-key.
W: GPG error: copy:/<<BUILDDIR>>/resolver-uIDqUN/apt_archive ./ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY

-- full extract from sbuild log --

...
dpkg-scanpackages: info: Wrote 1 entries to output Packages file.
gpg: keybox '/<<BUILDDIR>>/resolver-uIDqUN/gpg/pubring.kbx' created
gpg: /<<BUILDDIR>>/resolver-uIDqUN/gpg/trustdb.gpg: trustdb created
gpg: key 827E97084AD37184: public key "Sbuild Signer (Sbuild Build Dependency Archive Key) <email address hidden>" imported
gpg: Total number processed: 1
gpg: imported: 1
gpg: key 827E97084AD37184: "Sbuild Signer (Sbuild Build Dependency Archive Key) <email address hidden>" not changed
gpg: key 827E97084AD37184: secret key imported
gpg: Total number processed: 1
gpg: unchanged: 1
gpg: secret keys read: 1
gpg: secret keys imported: 1
gpg: using "Sbuild Signer" as default secret key for signing
Ign:1 copy:/<<BUILDDIR>>/resolver-uIDqUN/apt_archive ./ InRelease
Get:2 copy:/<<BUILDDIR>>/resolver-uIDqUN/apt_archive ./ Release [957 B]
Get:3 copy:/<<BUILDDIR>>/resolver-uIDqUN/apt_archive ./ Release.gpg [370 B]
Ign:3 copy:/<<BUILDDIR>>/resolver-uIDqUN/apt_archive ./ Release.gpg
Get:4 copy:/<<BUILDDIR>>/resolver-uIDqUN/apt_archive ./ Sources [349 B]
Get:5 copy:/<<BUILDDIR>>/resolver-uIDqUN/apt_archive ./ Packages [431 B]
Fetched 2107 B in 0s (19.4 kB/s)
Reading package lists...
W: copy:///<<BUILDDIR>>/resolver-uIDqUN/apt_archive/./Release.gpg: The key(s) in the keyring /etc/apt/trusted.gpg.d/sbuild-build-depends-archive.gpg are ignored as the file is not readable by user '_apt' executing apt-key.
W: GPG error: copy:/<<BUILDDIR>>/resolver-uIDqUN/apt_archive ./ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 827E97084AD37184
Reading package lists...

Install core build dependencies (apt-based resolver)
...