Fix a bug in using TLES env variable

[pnuu]

The behaviour of using TLES environment variable was changed in #129. This PR reverts the usage so that TLES='/path/to/tle_files/tle*txt" works again.

The question is, should the TLES=/path/to/tle_files" introduced in #129 work?

• [x] Tests adjusted
• [x] Tests passed
• [x] Fully documented
• [x] Passes flake8 pyorbital

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (2c06303) 87.51% compared to head (97df4c1) 87.51%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #143 +/- ## ======================================= Coverage 87.51% 87.51% ======================================= Files 14 14 Lines 2210 2210 ======================================= Hits 1934 1934 Misses 276 276 ``` | [Flag](https://app.codecov.io/gh/pytroll/pyorbital/pull/143/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pytroll/pyorbital/pull/143/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `87.51% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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[coveralls]

coverage: 87.562%. remained the same when pulling 97df4c178ddcb00580b3ed9170597c0e3e990382 on pnuu:bugfix-local-tles into 2c063038c119d091a6302da6061fd7863108d1ae on pytroll:main.

[mraspaud]

It's actually a feature, not a bug... but the new behaviour in 1.8 is actually problematic, so we are reverting this here.