Add get_beamline_based_on_env_variable

[Tom-Willemsen]

Suggested in DiamondLightSource/hyperion#871 (comment)

Useful for hyperion, update PR at DiamondLightSource/hyperion#871 to use this once this is merged here.

[codecov]

Codecov Report

Merging #166 (4592b4f) into main (eed89e7) will decrease coverage by 1.45%.
Report is 63 commits behind head on main.
The diff coverage is 85.38%.

@@            Coverage Diff             @@
##             main     #166      +/-   ##
==========================================
- Coverage   90.33%   88.89%   -1.45%     
==========================================
  Files          64       67       +3     
  Lines        2122     2332     +210     
==========================================
+ Hits         1917     2073     +156     
- Misses        205      259      +54     

Files Changed	Coverage Δ
src/dodal/beamlines/i03.py	87.14% <ø> (ø)
src/dodal/devices/aperturescatterguard.py	73.21% <ø> (ø)
src/dodal/devices/oav/oav_calculations.py	100.00% <ø> (ø)
src/dodal/devices/oav/oav_detector.py	97.36% <ø> (-0.20%)	⬇️
src/dodal/devices/oav/oav_parameters.py	93.82% <ø> (ø)
...l/devices/oav/pin_image_recognition/manual_test.py	0.00% <0.00%> (ø)
src/dodal/log.py	98.78% <ø> (ø)
...c/dodal/devices/oav/pin_image_recognition/utils.py	74.03% <74.03%> (ø)
src/dodal/beamlines/i23.py	88.23% <88.23%> (ø)
src/dodal/devices/i23/gonio.py	92.30% <92.30%> (ø)
... and 8 more

... and 7 files with indirect coverage changes

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[DominicOram]

Minor nit, take it or leave it