GH-47486: [Dev][R] Define default R_UPDATE_CLANG (#47487)

### Rationale for this change

Warnings emitted on Docker build

### What changes are included in this PR?

Initialise R_UPDATE_CLANG to false

### Are these changes tested?

Nope

### Are there any user-facing changes?

Nope
* GitHub Issue: #47486

Authored-by: Nic Crane <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: thisisnic

.env

@@ -83,6 +83,7 @@ R_ORG=rhub
 R_TAG=latest
 
 # Env vars for R builds
+R_UPDATE_CLANG=false
 R_CUSTOM_CCACHE=false
 ARROW_R_DEV=TRUE
 R_PRUNE_DEPS=FALSE