[R] Clean up environment variables in build scripts

[nealrichardson]

Describe the enhancement requested

There is some drift/inconsistency/confusion in the various env vars we expose to control the R package build. One result of this is that we have crossbow jobs that are downloading nightly binaries instead of building from source. See discussion leading up to and including #39602 (comment).

Proposal:

• FORCE_BUNDLED_BUILD should be deprecated in favor of LIBARROW_BUILD. I don't think we recommend it to people, it's more for our testing, and it's not named accurately anyway--it doesn't force a "bundled build", it routes you to nixlibs.R, where you'll either download a binary or get the bundled source build. Another way you can get configure to skip looking for libarrow on the system should be LIBARROW_BINARY equal to something other than false or unset--you're asking for a specific libarrow binary. (And today you can still get the same effect as FORCE_BUNDLED_BUILD by setting ARROW_USE_PKG_CONFIG=false and not having ARROW_HOME set.)
• LIBARROW_BUILD=true should entail LIBARROW_BINARY=false. If LIBARROW_BUILD=true and LIBARROW_BINARY is something other than unset or false, raise an error.
• Set download_libarrow_ok based on LIBARROW_BINARY=false, not LIBARROW_DOWNLOAD. Remove references to LIBARROW_DOWNLOAD. (Confirm if this download_libarrow_ok is necessary, or if LIBARROW_BINARY=false is already handled correctly.)
• If we want something explicit about preventing downloading, that's TEST_OFFLINE_BUILD (as of GH-39601: [R] Don't download cmake when TEST_OFFLINE_BUILD=true #39602, it blocks all downloading). If that's a feature we want to advertise, we could rename that something that's not TEST_.
• Also fix the ubuntu-r-only-r job, and audit other jobs that use these env vars. (This may be covered by GH-39624: [R][CI] Add CMake to docker file and update envvars #39625)

Component(s)

R

[assignUser]

Sounds good, thanks for the summary and raising the issue!