[Feat] Add MMCV_BASE_URL env variable to support customize mmcv find link

[ice-tong]

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Motivation

Add MMCV_FIND_BASE_URL environment variable to support customize mmcv find link.

Modification

• mim/commands/install.py
• mim/utils/default.py
• mim/utils/__init__.py

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

>  MMCV_BASE_URL=https://download.openmmlab.com mim install mmcv-full

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #148 (ac219a5) into main (eb5c5c6) will increase coverage by 21.90%.
The diff coverage is 69.86%.

@@             Coverage Diff             @@
##             main     #148       +/-   ##
===========================================
+ Coverage   32.58%   54.48%   +21.90%     
===========================================
  Files          21       23        +2     
  Lines        1593     1562       -31     
  Branches      363      333       -30     
===========================================
+ Hits          519      851      +332     
+ Misses       1032      584      -448     
- Partials       42      127       +85     

Flag	Coverage Δ
unittests	54.48% <69.86%> (+21.90%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mim/version.py	0.00% <0.00%> (ø)
mim/click/compat.py	45.45% <45.45%> (ø)
mim/click/option.py	54.05% <50.00%> (+23.75%)	⬆️
mim/utils/utils.py	67.88% <57.14%> (+17.07%)	⬆️
mim/commands/install.py	71.97% <70.66%> (+3.33%)	⬆️
mim/commands/uninstall.py	76.19% <76.47%> (-16.12%)	⬇️
mim/click/__init__.py	100.00% <100.00%> (ø)
mim/commands/download.py	88.46% <100.00%> (+53.84%)	⬆️
mim/commands/search.py	54.00% <100.00%> (+41.24%)	⬆️
mim/utils/__init__.py	100.00% <100.00%> (ø)
... and 12 more

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