Fix for dash shell

[rbt]

As mentioned in #6 the Debian shell fails to execute install-on-perl.sh because it doesn't have the set -o pipefail option.

set -o outputs a list of options available, so simply look to see if that option is supported before trying to set it.

*** /tmp/orig   2020-07-08 20:30:08.494403369 -0400
--- /tmp/install-on-perl.sh 2020-07-08 20:30:21.810601543 -0400
***************
*** 1,7 ****
  #!/usr/bin/env sh

  set -o errexit
! set -o pipefail

  for cmd in "rakudobrew", "p6env"; do
      if command -v $cmd >/dev/null 2>&1 ; then
--- 1,9 ----
  #!/usr/bin/env sh

  set -o errexit
! if [ "`set -o | grep 'pipefail'`" != "" ]; then
!   set -o pipefail
! fi

  for cmd in "rakudobrew", "p6env"; do
      if command -v $cmd >/dev/null 2>&1 ; then

[patrickbkr]

This is actually fixed in 899c8c8e41, but isn't yet deployed to the website. I'm looking into speeding up the deployment process. Sorry for the inconvenience! Closing as this actually is already fixed.