Make AWS Codebuild instructions simpler.

No need to manually record PWD and to change back to previous directory if you
let a subshell do it for you.

Fixes #458

Author: adk-swisstopo

README.md

@@ -42,9 +42,7 @@ The following snippet can be inserted in your buildspec.yml (or buildspec defini
 **Side Note:** if you use the below steps, please unset your golang version in the buildspec and run the installer manually.
 
 ```yaml
-- BUILD_DIR=$PWD
-- cd /root/.goenv/plugins/go-build/../.. && git pull && cd -
-- cd $BUILD_DIR
+- (cd /root/.goenv/plugins/go-build/../.. && git pull)
 ```
 
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