Updated regex to string, populate-fixtures.sh

I added quotes around the regex on line 53 of
tests/scripts/populate-fixtures.sh. I did this as I received an error
running 'make populate' in the fabri-sdk-go directory, "test/scripts/populate-fixtures.sh:
line 53: unexpected argument `(' to conditional binary operator" on
MINGW64 with Windows 10. The error prevents vendor packages from
populating. With the quotes around the expression the vendor packages
download correctly. A seperate patch for populate-vendors.sh is necessary
with this change request.
See: https://gerrit.hyperledger.org/r/#/c/26919/

Change-Id: I97ed8cdd389d6e868c2b65feba1be067389870f1
Signed-off-by: JVSCLP <jvs@clpinc.com>

test/scripts/populate-fixtures.sh

@@ -50,7 +50,8 @@ function recordCryptoCacheResult {
 
 function isScriptCurrent {
     declare filesModified=$(git diff --name-only --diff-filter=ACMRTUXBD HEAD | tr '\n' ' ' | xargs)
-    if [[ "${filesModified}" =~ ( |^)(test/scripts/populate-fixtures.sh)( |$) ]]; then
+    declare matcher='( |^)(test/scripts/populate-fixtures.sh)( |$)'
+    if [[ "${filesModified}" =~ ${matcher} ]]; then
         echo "Fixtures script modified - will need to repopulate fixtures"
         return 1
     fi