Swarming: Avoids mounting volume (#5213)

## Overview
Swarming, as well as K8S has issues when trying to mount this volume to
the container.
```bash
mount /mnt/scratch0 -o remount,exec,suid,dev
```
So we added a validation to not mount when we are either in K8S or in
swarming.

Note: Uses SWARMING_BOT env var as its the one we [already use in the
swarming
request](https://github.com/google/clusterfuzz/blob/master/src/clusterfuzz/_internal/swarming/__init__.py#L134)

## Tests performed
I created a custom bash script to test this:
```bash
is_truthy() {
  [[ "$1" =~ ^([Tt]rue|1)$ ]]
}

should_mount() {
  if is_truthy "$IS_K8S_ENV" || is_truthy "$SWARMING_BOT"; then
    return 1
  fi
  return 0
}

test_mount() {
  unset IS_K8S_ENV SWARMING_BOT
  [[ "$1" != "unset" ]] && export IS_K8S_ENV=$1
  [[ "$2" != "unset" ]] && export SWARMING_BOT=$2
  printf "IS_K8S_ENV=%-5s | SWARMING_BOT=%-5s  =>" "$1" "$2"
  if should_mount; then echo " [MOUNT EXECUTED]"; else echo " [MOUNT SKIPPED]"; fi
}
'
```
Against multiple inputs:
```bash
echo "=== DRY RUN OUTPUT USING is_truthy() ==="
test_mount "unset" "unset"
test_mount "1"     "unset"
test_mount "true"  "unset"
test_mount "false" "unset"
test_mount "False" "unset"
test_mount "0"     "unset"
test_mount "unset" "True"
test_mount "unset" "1"
test_mount "unset" "false"
test_mount "false" "false"
```
And got this results:
```bash
=== DRY RUN OUTPUT USING is_truthy() ===
IS_K8S_ENV=unset | SWARMING_BOT=unset  => [MOUNT EXECUTED]
IS_K8S_ENV=1     | SWARMING_BOT=unset  => [MOUNT SKIPPED]
IS_K8S_ENV=true  | SWARMING_BOT=unset  => [MOUNT SKIPPED]
IS_K8S_ENV=false | SWARMING_BOT=unset  => [MOUNT EXECUTED]
IS_K8S_ENV=False | SWARMING_BOT=unset  => [MOUNT EXECUTED]
IS_K8S_ENV=0     | SWARMING_BOT=unset  => [MOUNT EXECUTED]
IS_K8S_ENV=unset | SWARMING_BOT=True   => [MOUNT SKIPPED]
IS_K8S_ENV=unset | SWARMING_BOT=1      => [MOUNT SKIPPED]
IS_K8S_ENV=unset | SWARMING_BOT=false  => [MOUNT EXECUTED]
IS_K8S_ENV=false | SWARMING_BOT=false  => [MOUNT EXECUTED]

```

Author: IvanBM18

docker/base/setup_common.sh

@@ -33,10 +33,22 @@ then
   export PREEMPTIBLE=True
 fi
 
+is_truthy() {
+  [[ "$1" =~ ^([Tt]rue|1)$ ]]
+}
+
 # Make sure mounted volume doesn't have noexec,nosuid,nodev
-# Running this in k8s environment will cause mounting errors
-if [[ -z "$IS_K8S_ENV" ]]
-then
+# Running this in k8s or swarming environment will cause mounting errors
+should_mount() {
+  # Skip if running in Kubernetes or as a Swarming Job
+  if is_truthy "$IS_K8S_ENV" || is_truthy "$SWARMING_BOT"; then
+    return 1
+  fi
+
+  return 0
+}
+
+if should_mount; then
   mount /mnt/scratch0 -o remount,exec,suid,dev
 fi