Bug#948731: [regression] sage -v doesn't work again

[John Scott]

Package: sagemath-common
Version: 8.9-3
Severity: normal
Control: affects -1 cantor-backend-sage

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I previously reported this as #923166 and it was fixed in 8.6-6. It seems
it's come back: sage -v says
/usr/bin/sage: line 16: /src/bin/sage-version.sh: No such file or directory

I noticed this trying to use Cantor with Sage as I did then. Fortunately
the new version of Cantor doesn't crash anymore and says instead
 Failed to determine the version of Sage. Please check your installation and
 the output of 'sage -v'.

I thought this worked fine with 8.9-2, but maybe not.
The cause seems to be that Sage's root isn't set: sage --root gives no output.

Unfortunately I'm not familiar with it yet, otherwise I would like to help set
up an autopkgtest. However, 'sage -v' returns 0, so maybe the shebang
should be changed to
 #!/usr/bin/env bash -e
to catch errors.

- -- System Information:
Debian Release: bullseye/sid
  APT prefers testing-debug
  APT policy: (500, 'testing-debug'), (500, 'testing'), (2, 'unstable'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 5.3.0-3-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE= (charmap=UTF-8)
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages sagemath-common depends on:
ii  python3  3.7.5-3

sagemath-common recommends no packages.

sagemath-common suggests no packages.

- -- no debconf information

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