Hi Constantin,
Ohh, interesting!
r-lib, R-devel, Rtools 35
My first test with R-devel using r-lib/actions/setup-r and the default Rtools 35 did install R but failed later with multiple instances of the error:
sh: c:/rtools40/mingw32/bin/gcc: No such file or directory
GHA log and GHA workflow yml. That's when I read this thread and started my https://github.com/lcolladotor/actions/tree/support_win_rtools_40_edit and GitHub - lcolladotor/actions at support_win_rtools_40_edit_repo_config tests.
Now in the day with more light I see that it was calling a path with rtools40 already 
2020-04-19T04:04:30.3668057Z Run a test with windows (r-devel) just to see how that goes
2020-04-19T04:04:30.3928899Z ##[group]Run r-lib/actions/setup-r@master
2020-04-19T04:04:30.3929052Z with:
2020-04-19T04:04:30.3929194Z r-version: devel
2020-04-19T04:04:30.3929260Z rtools-version: 35
2020-04-19T04:04:30.3929338Z Ncpus: 1
2020-04-19T04:04:30.3929418Z crayon.enabled: TRUE
2020-04-19T04:04:30.3929499Z remove-openmp-macos: true
2020-04-19T04:04:30.3929564Z env:
2020-04-19T04:04:30.3929645Z R_REMOTES_NO_ERRORS_FROM_WARNINGS: true
2020-04-19T04:04:30.3929733Z RSPM:
2020-04-19T04:04:30.3929810Z BIOCVERSIONNUM: 3.11
2020-04-19T04:04:30.3929873Z ##[endgroup]
2020-04-19T04:04:30.7892723Z [command]choco install qpdf --no-progress
2020-04-19T04:04:31.3248441Z [command]d:\a\_temp\Rtools35.exe /VERYSILENT /SUPPRESSMSGBOXES
2020-04-19T04:04:32.9170557Z Chocolatey v0.10.15
2020-04-19T04:04:33.6341189Z [command]d:\a\_temp\R-devel-win.exe /VERYSILENT /SUPPRESSMSGBOXES /DIR=C:\R
From the full logs I see that it was installing Rtools 35 though.
New test
I just submitted a new test using R-devel with Rtools 35, like you did here.
Though now I'm running into a new unrelated error:
Error in utils::download.file(url, path, method = download_method(), quiet = quiet, :
cannot open URL 'http://crandeps.r-pkg.org/deps/recount3,knitr,rmarkdown,BiocStyle,knitcitations,sessioninfo,testthat,RefManageR,covr,SummarizedExperiment,BiocFileCache,AnnotationHub'
Calls: cat ... get_cran_deps -> download_json -> download -> <Anonymous>
In addition: Warning message:
In utils::download.file(url, path, method = download_method(), quiet = quiet, :
cannot open URL 'https://crandeps.r-pkg.org:443/deps/recount3,knitr,rmarkdown,BiocStyle,knitcitations,sessioninfo,testthat,RefManageR,covr,SummarizedExperiment,BiocFileCache,AnnotationHub': HTTP status was '404 Not Found'
Execution halted
##[error]Process completed with exit code 1.
Jeroen Ooms
I see that my fork at GitHub - lcolladotor/actions: GitHub Actions for the R community is 1 commit behind master... and that commit is by Jeroen Ooms and Jim Hester, where they add the code for Rtools 40 for Windows Use rtools40 for R >= 4.0 · r-lib/actions@4d14c61 · GitHub hehe.
Best,
Leo