[ci] [R-package] Add period after specified linter names in nolint comments

[nicklamiller]

Fixes: #6948

Adds periods after specified linter names in nolint comments so that only those linters are ignored. This was fairly straightforward, just a couple things to note:

• In a couple places square brackets were used for linters e.g. [unused_import]. For these, I've removed the square brackets which is consistent with how linters are specified in the majority of other nolint comments.
• Almost all lines without the trailing period only violated their specified linter so didn't have to change code or specify extra linters (with the exception of two lines that require("processx")).

Made sure there are no other nolint lines without the trailing period with:

grep -R "nolint:" . | grep -v '\.$'

[jameslamb]

Thanks for doing this and for correcting my understanding of how {lintr} works!