ARROW-7665: [R] Build in parallel in linuxLibs.R
ARROW-7665: [R] Build in parallel in linuxLibs.R

Also fix the dependency relationship between Bison and Flex

Closes #6271 from pitrou/ARROW-7665-faster-r-docker and squashes the following commits:

e54f14772 <Antoine Pitrou> Remove flex build changes
7de0f1524 <Antoine Pitrou> Address review comments
3fc6cb62f <Antoine Pitrou> ARROW-7665:  Build in parallel in linuxLibs.R

Authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: Neal Richardson <beb59f0141269d2d04e73d0dcc1967b64ba87e13@gmail.com>

kou/arrow,wesm/arrow,cpcloud/arrow,apache/arrow,icexelloss/arrow,xhochy/arrow,laurentgo/arrow,xhochy/arrow,icexelloss/arrow,apache/arrow,icexelloss/arrow,xhochy/arrow,laurentgo/arrow,apache/arrow,laurentgo/arrow,apache/arrow,laurentgo/arrow,apache/arrow,apache/arrow,cpcloud/arrow,wesm/arrow,wesm/arrow,kou/arrow,xhochy/arrow,cpcloud/arrow,cpcloud/arrow,wesm/arrow,laurentgo/arrow,wesm/arrow,xhochy/arrow,cpcloud/arrow,xhochy/arrow,icexelloss/arrow,laurentgo/arrow,cpcloud/arrow,xhochy/arrow,icexelloss/arrow,cpcloud/arrow,apache/arrow,apache/arrow,cpcloud/arrow,laurentgo/arrow,kou/arrow,kou/arrow,kou/arrow,laurentgo/arrow,xhochy/arrow,wesm/arrow,xhochy/arrow,icexelloss/arrow,cpcloud/arrow,icexelloss/arrow,laurentgo/arrow,xhochy/arrow,laurentgo/arrow,kou/arrow,kou/arrow,kou/arrow,cpcloud/arrow,icexelloss/arrow,kou/arrow,apache/arrow,icexelloss/arrow,icexelloss/arrow,laurentgo/arrow,icexelloss/arrow,xhochy/arrow,wesm/arrow,cpcloud/arrow,cpcloud/arrow,laurentgo/arrow,apache/arrow,kou/arrow,xhochy/arrow,wesm/arrow,wesm/arrow,icexelloss/arrow,kou/arrow,wesm/arrow,xhochy/arrow,kou/arrow,icexelloss/arrow,cpcloud/arrow,laurentgo/arrow,xhochy/arrow,kou/arrow,laurentgo/arrow,apache/arrow,apache/arrow,icexelloss/arrow,xhochy/arrow,cpcloud/arrow,wesm/arrow,apache/arrow,wesm/arrow,wesm/arrow,wesm/arrow,kou/arrow,laurentgo/arrow,apache/arrow