GH-47125: [CI][Dev] Fix shellcheck errors in the ci/scripts/integration_hdfs.sh (#47126)

### Rationale for this change

This is the sub issue #44748.

* SC2034: source_dir appears unused. Verify use (or export if used externally).
* SC2086: Double quote to prevent globbing and word splitting.
* SC2155: Declare and assign separately to avoid masking return values.

```
shellcheck ci/scripts/integration_hdfs.sh

In ci/scripts/integration_hdfs.sh line 22:
source_dir=${1}/cpp
^--------^ SC2034 (warning): source_dir appears unused. Verify use (or export if used externally).

In ci/scripts/integration_hdfs.sh line 25:
export CLASSPATH=$($HADOOP_HOME/bin/hadoop classpath --glob)
       ^-------^ SC2155 (warning): Declare and assign separately to avoid masking return values.
                   ^----------^ SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean:
export CLASSPATH=$("$HADOOP_HOME"/bin/hadoop classpath --glob)

In ci/scripts/integration_hdfs.sh line 45:
pushd ${build_dir}
      ^----------^ SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean:
pushd "${build_dir}"

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- source_dir appears unused. Verify...
  https://www.shellcheck.net/wiki/SC2155 -- Declare and assign separately to ...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...
```

### What changes are included in this PR?

* SC2034: disable shellcheck
* SC2086: Quote variables.
* SC2155: separate variable declaration and export.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #47125

Authored-by: Hiroyuki Sato <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: hiroyuki-sato

.pre-commit-config.yaml

@@ -331,6 +331,7 @@ repos:
           ?^ci/scripts/integration_arrow_build\.sh$|
           ?^ci/scripts/integration_arrow\.sh$|
           ?^ci/scripts/integration_dask\.sh$|
+          ?^ci/scripts/integration_hdfs\.sh$|
           ?^ci/scripts/integration_spark\.sh$|
           ?^ci/scripts/matlab_build\.sh$|
           ?^ci/scripts/msys2_system_clean\.sh$|

ci/scripts/integration_hdfs.sh

@@ -19,10 +19,12 @@
 
 set -e
 
+# shellcheck disable=SC2034
 source_dir=${1}/cpp
 build_dir=${2}/cpp
 
-export CLASSPATH=$($HADOOP_HOME/bin/hadoop classpath --glob)
+HADOOP_CLASSPATH=$("$HADOOP_HOME/bin/hadoop" classpath --glob)
+export CLASSPATH="${HADOOP_CLASSPATH}"
 export HADOOP_CONF_DIR=$HADOOP_HOME/etc/hadoop
 export LIBHDFS3_CONF=$HADOOP_CONF_DIR/hdfs-site.xml
 export ARROW_LIBHDFS3_DIR=$CONDA_PREFIX/lib
@@ -42,7 +44,7 @@ function use_libhdfs_dir() {
 
 # execute cpp tests
 export ARROW_HDFS_TEST_LIBHDFS_REQUIRE=ON
-pushd ${build_dir}
+pushd "${build_dir}"
 
 debug/arrow-io-hdfs-test
 debug/arrow-hdfs-test