sagemathgh-36364: Do not run sage-env more than once, for real.
    
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Do not run sage-env more than once.

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In the previous version of sage-env, in the lines
```
SAGE_ENV_VERSION="6:$SAGE_LOCAL:$SAGE_VENV:$SAGE_SRC"
if [ "$SAGE_ENV_SOURCED" = "$SAGE_ENV_VERSION" ]; then
    # Already sourced, nothing to do.
    return 0
fi
export SAGE_ENV_SOURCED="$SAGE_ENV_VERSION"
```
the test would fail the first time, as it should, but when the rest of
the file was executed, SAGE_SRC would get changed. That meant that the
test would fail the next time and so the file would get executed again.
We move the last line to the end of the file, and also change it to
explicitly use `SAGE_LOCAL`, `SAGE_VENV`, and `SAGE_SRC` so that it uses
the up-to-date versions.

This should fix part of the problem in sagemath#36337: it should prevent
duplicate `-rpath` entries which were arising because sage-env was
getting executed twice.

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### ⌛ Dependencies

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URL: sagemath#36364
Reported by: John H. Palmieri
Reviewer(s): Dima Pasechnik, John H. Palmieri, Matthias Köppe