[ci] use newer h5py in AppVeyor jobs (fixes #5995)

[jameslamb]

Fixes #5995.

Attempts to force conda to use a newer version of h5py in tests, to avoid the errors in the linked issue related to the combination of newer numpy and older h5py being incompatible.

Also fixes the following error caused by the new release of {lintr} a few hours ago:

[1] Total linting issues found: 1
[[1]]
/home/runner/work/LightGBM/LightGBM/R-package/tests/testthat/test_dataset.R:449:16: warning: [paste] strrep(x, times) is better than paste0(rep(x, times), collapse = "").
long_name <- paste0(rep("a", 1000L), collapse = "")
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

(failed R linting job)

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.