ARROW-17166: [R] [CI] force_tests() cannot return TRUE (#13680)

OOMing no longer occurs with large memory tests, so this PR no longer removes those. During the investigation, I uncovered a bug with `force_tests()` - which became the aim of the PR.  
~It looks like setting the docker `TZ` to `"MART"` triggers a fail in the Ubuntu CI jobs.~
~It turns out I misinterpreted the CI output and this was an issue with OOM in the large memory tests. See [comment](#13680 (comment)

Authored-by: Dragoș Moldovan-Grünfeld <[email protected]>
Signed-off-by: Jonathan Keane <[email protected]>

r/tests/testthat/helper-skip.R

@@ -22,7 +22,7 @@ build_features <- c(
 )
 
 force_tests <- function() {
-  identical(tolower(Sys.getenv("ARROW_R_force_tests()")), "true")
+  identical(tolower(Sys.getenv("ARROW_R_FORCE_TESTS")), "true")
 }
 
 skip_if_not_available <- function(feature) {