fix(rust): use ERE-compatible regex in start_release.sh version check

The version check used the PCRE escape \d in an egrep pattern:

  REGEX_VERSION='^\d+\.\d+\.\d+$'
  MATCHES=$(echo "$1" | egrep $REGEX_VERSION | wc -l)

egrep uses POSIX Extended Regular Expressions, which do not support \d.
The pattern therefore matched nothing, every input was treated as
invalid, and the script always exited with:

  Version "X.Y.Z" must be N.N.N

This caused the 'Run start_release.sh and open a release PR' job to fail
with exit code 1 even though VERSION=1.3.0 had already passed the
workflow's own pre-check (which uses bash regex ^[0-9]+\.[0-9]+\.[0-9]+$).

Replace \d with [0-9] so the regex actually matches N.N.N, and quote the
variable expansion in egrep for safety.

Author: josecorella

DynamoDbEncryption/runtimes/rust/start_release.sh

@@ -9,9 +9,11 @@ fi
 # Go to the directory of this script
 cd $( dirname ${BASH_SOURCE[0]} )
 
-# Check if the provided argument matches the version pattern
-REGEX_VERSION='^\d+\.\d+\.\d+$'
-MATCHES=$(echo "$1" | egrep $REGEX_VERSION | wc -l)
+# Check if the provided argument matches the version pattern.
+# Note: egrep uses POSIX ERE, which does not support PCRE escapes like \d.
+# Use [0-9] instead so the regex actually matches N.N.N versions.
+REGEX_VERSION='^[0-9]+\.[0-9]+\.[0-9]+$'
+MATCHES=$(echo "$1" | egrep "$REGEX_VERSION" | wc -l)
 if [ $MATCHES -eq 0 ]; then
    echo 1>&2 "Version \"$1\" must be N.N.N"
    exit 1