Capture subshell output for OiCS

[modular-magician]

I'd tested feeding variables in & error cases, but not actually the output. Editing them inside the subshell doesn't make it's way back to the parent. Capture the output from the subshell.

I get the output Diffs: TF OiCS: [Diff](https://github.com/modular-magician/docs-examples/compare/auto-pr-3050-old..auto-pr-3050) from the test script below:

set -e

PR_NUMBER=3050
NEW_BRANCH=auto-pr-$PR_NUMBER
OLD_BRANCH=auto-pr-$PR_NUMBER-old
TFOICS_SCRATCH_PATH=https://modular-magician:$GITHUB_TOKEN@github.com/modular-magician/docs-examples
TFOICS_LOCAL_PATH=$PWD/../tfoics

set +e
DIFFS="Diffs: "
VAR=$(bash -e <<TRY
    # TF OICS
    mkdir -p $TFOICS_LOCAL_PATH
    git clone -b $NEW_BRANCH $TFOICS_SCRATCH_PATH $TFOICS_LOCAL_PATH
    pushd $TFOICS_LOCAL_PATH > /dev/null
    git fetch origin $OLD_BRANCH
    if ! git diff --exit-code --quiet origin/$NEW_BRANCH origin/$OLD_BRANCH; then
        echo "TF OiCS: [Diff](https://github.com/modular-magician/docs-examples/compare/$OLD_BRANCH..$NEW_BRANCH)"
    fi
    popd > /dev/null
TRY
)
if [ $? -ne 0 ]; then
  echo caught exception
else
DIFFS="$DIFFS $VAR"
fi
set -e
echo $DIFFS

Release Note Template for Downstream PRs (will be copied)

Derived from GoogleCloudPlatform/magic-modules#3054

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