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Update bash variables to use brace expansion, as it's more versatile and would
make things more consistent if/when we use more fancy bash in the future.

Reference: https://stackoverflow.com/questions/8748831/when-do-we-need-curly-braces-around-shell-variables

Bug: 936694
Change-Id: Iafcefd60405ac9564a7b36af893fdb5d72e6893d
Reviewed-on: https://chromium-review.googlesource.com/c/1493207
Auto-Submit: Darwin Huang <huangdarwin@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636338}