Bundle our own R 4.5.3 instead of building against Arch's R

Arch ships R 4.6.1, but JASP pins its R packages against a frozen CRAN
snapshot (2026-03-31 for 0.98.1) that targets R 4.5. Configuring against
Arch's R fails at Rcpp 1.1.1, which cannot compile on R 4.6 since
R_NamespaceRegistry was removed; the Rcpp release that fixes it is newer
than the pinned snapshot, so the lockfile cannot reach it either.

Build R 4.5.3 into /opt/jasp-desktop/R and point R_HOME at it, matching
what upstream CI does. This drops 'r' from depends entirely, so an Arch R
upgrade can no longer break an installed JASP, and the rebuild triggers
become Qt6 and ICU - the libraries actually still linked against.

.gitea/workflows/build.yml

@@ -70,6 +70,8 @@ jobs:
           df -h /
 
       - name: Build JASP in an Arch container
+        env:
+          GH_TOKEN: ${{ secrets.GH_TOKEN }}
         run: |
           docker rm -f jasp-build 2>/dev/null || true
           docker run --rm --name jasp-build \
@@ -77,6 +79,7 @@ jobs:
             -e JASP_TAG="${{ needs.check.outputs.jasp_tag }}" \
             -e OUT_DIR=/work/out \
             -e JOBS=5 \
+            -e GITHUB_PAT="${GH_TOKEN}" \
             archlinux:base-devel \
             bash /work/build/build-jasp.sh