Allowing the use of /bin/bash by tox

tox on certain platforms will complain that /bin/bash is not allowed to
be used.  I'm allowing it to be used with this change.

tox.ini

@@ -8,6 +8,7 @@ passenv = DBT_* PYTEST_ADDOPTS
 deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/dev-requirements.txt
+allowlist_externals=/bin/bash
 
 [testenv:integration-spark-databricks-http]
 basepython = python3.8
@@ -17,6 +18,7 @@ deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/dev-requirements.txt
     -e.
+allowlist_externals=/bin/bash
 
 [testenv:integration-spark-databricks-odbc-cluster]
 basepython = python3.8
@@ -27,6 +29,7 @@ deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/dev-requirements.txt
     -e.
+allowlist_externals=/bin/bash
 
 [testenv:integration-spark-databricks-odbc-sql-endpoint]
 basepython = python3.8
@@ -37,6 +40,7 @@ deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/dev-requirements.txt
     -e.
+allowlist_externals=/bin/bash
 
 
 [testenv:integration-spark-thrift]
@@ -48,6 +52,7 @@ deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/dev-requirements.txt
     -e.
+allowlist_externals=/bin/bash
 
 [testenv:integration-spark-session]
 basepython = python3
@@ -60,3 +65,4 @@ deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/dev-requirements.txt
     -e.[session]
+allowlist_externals=/bin/bash