Switch to RSPM snapshot for legacy package versions (MRAN is dead)

The closest frozen date to MRAN's 2018-05-01 is RSPM's 2018-05-03.
The only dependency that was updated in these two days was stringi
1.2.2 on 2018-05-02

https://cran.r-project.org/src/contrib/Archive/stringi/

data/legacy-package-versions.csv

@@ -17,7 +17,7 @@ yaml,2.1.19,archive
 backports,1.1.2,archive
 R.methodsS3,1.7.1,archive
 glue,1.2.0,archive
-stringi,1.1.7,archive
+stringi,1.2.2,archive
 curl,3.2,archive
 openssl,1.0.1,archive
 base64enc,0.1-3,current

scripts/download-legacy-versions.R

@@ -4,7 +4,7 @@
 # defined in .github/workflows/legacy.yaml
 
 output <- "data/legacy-package-versions.csv"
-snapshot <- "2018-05-01"
+snapshot <- "2018-05-03"
 
 suppressPackageStartupMessages({
   library(igraph)
@@ -13,7 +13,7 @@ suppressPackageStartupMessages({
 
 # Download available package versions from MRAN snapshot
 # https://mran.microsoft.com/timemachine
-mranUrl <- sprintf("https://cran.microsoft.com/snapshot/%s/", snapshot)
+mranUrl <- sprintf("https://packagemanager.posit.co/cran/%s", snapshot)
 
 mran <- available.packages(contrib.url(mranUrl, "source"))