CI: tasks use bundle exec to run

Fixes
```
+ rake build:aws
rake aborted!
LoadError: cannot load such file -- aws-sdk-s3 (LoadError)
```

Author: aramprice

ci/tasks/aggregate-stemcells/run

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
-
-set -ex
+set -eu -o pipefail
+set -x
 
 AWS_CREDS_FILE=~/.aws/credentials
 mkdir -p ~/.aws
@@ -11,9 +11,10 @@ aws_access_key_id = ${AWS_ACCESS_KEY}
 aws_secret_access_key = ${AWS_SECRET_KEY}
 EOF
 
-cd stemcell-builder
-bundle install
-rake build:aws_aggregate
+pushd stemcell-builder
+  bundle install
+  bundle exec rake build:aws_aggregate
 
-mv bosh-windows-stemcell/*.tgz ../bosh-windows-stemcell
-mv bosh-windows-stemcell/*.sha ../sha
+  mv bosh-windows-stemcell/*.tgz ../bosh-windows-stemcell
+  mv bosh-windows-stemcell/*.sha ../sha
+popd

ci/tasks/copy-aws-stemcell/run

@@ -21,7 +21,7 @@ fi
 
 pushd stemcell-builder
   bundle install
-  rake build:aws_ami
+  bundle exec rake build:aws_ami
 
   mv copied-regional-stemcells/*.tgz ../copied-regional-stemcells
 popd

ci/tasks/create-aws-stemcell/run

@@ -34,7 +34,7 @@ pushd stemcell-builder
   cp "${CONCOURSE_ROOT}/psmodules-zip-output/bosh-psmodules.zip" "${CONCOURSE_ROOT}/bosh-agent/agent.zip" build
 
   bundle install
-  rake build:aws
+  bundle exec rake build:aws
 
   mv bosh-windows-stemcell/* ../bosh-windows-stemcell
 

ci/tasks/create-azure-stemcell/run

@@ -13,7 +13,7 @@ pushd stemcell-builder
   cp "${CONCOURSE_ROOT}/psmodules-zip-output/bosh-psmodules.zip" "${CONCOURSE_ROOT}/bosh-agent/agent.zip" build
 
   bundle install
-  rake build:azure
+  bundle exec rake build:azure
 
   mv bosh-windows-stemcell/* ../bosh-windows-stemcell
 

ci/tasks/create-gcp-stemcell/run

@@ -13,7 +13,7 @@ pushd stemcell-builder
   cp "${CONCOURSE_ROOT}/psmodules-zip-output/bosh-psmodules.zip" "${CONCOURSE_ROOT}/bosh-agent/agent.zip" build
 
   bundle install
-  rake build:gcp
+  bundle exec rake build:gcp
 
   mv bosh-windows-stemcell/* ../bosh-windows-stemcell
 

ci/tasks/label-aws-stemcell-for-production/run

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
-
-set -ex
+set -eu -o pipefail
+set -x
 
 AWS_CREDS_FILE=~/.aws/credentials
 mkdir -p ~/.aws
@@ -13,5 +13,5 @@ EOF
 
 pushd stemcell-builder
   bundle install
-  rake aws:label:for_production
+  bundle exec rake aws:label:for_production
 popd

ci/tasks/label-gcp-stemcell-for-production/run

@@ -1,7 +1,8 @@
 #!/usr/bin/env bash
+set -eu -o pipefail
+set -x
 
-set -ex
-
-cd stemcell-builder
-bundle install
-rake gcp:label:for_production
+pushd stemcell-builder
+  bundle install
+  bundle exec rake gcp:label:for_production
+popd

ci/tasks/publish-gcp-stemcell/run

@@ -2,10 +2,10 @@
 set -eu -o pipefail
 set -x
 
-cd stemcell-builder
-bundle install
-rake publish:gcp
-rake gcp:label:for_test
-cd ..
+pushd stemcell-builder
+  bundle install
+  bundle exec rake publish:gcp
+  bundle exec rake gcp:label:for_test
+popd
 
 mv bosh-windows-stemcell/*.tgz untested-stemcell

ci/tasks/wait-for-ami-availability/run

@@ -19,6 +19,6 @@ fi
 
 pushd stemcell-builder
   bundle install
-  rake build:validate_ami
-  rake aws:label:for_test
+  bundle exec rake build:validate_ami
+  bundle exec rake aws:label:for_test
 popd