fix difference in truststore env var name

[Alexjsenn]

Not sure when it was changed, but the name of the env var for the ssl truststore was changed in entrypoint, and was not updated in the certificatePostHandler.

another update needed

[andrewazores]

$ ag --ignore-dir web-client TRUSTSTORE_DIR
src/main/java/com/redhat/rhjmc/containerjfr/net/web/http/api/v2/CertificatePostHandler.java
78:    private static final String TRUSTSTORE_DIR = "TRUSTSTORE_DIR";
142:        if (!env.hasEnv(TRUSTSTORE_DIR)) {
146:        String truststoreDir = env.getEnv(TRUSTSTORE_DIR);

src/main/extras/app/entrypoint.sh
40:if [ -z "$SSL_TRUSTSTORE_DIR" ]; then
41:    SSL_TRUSTSTORE_DIR="/truststore"
44:export SSL_TRUSTSTORE_DIR
60:    if [ ! -d "$SSL_TRUSTSTORE_DIR" ]; then
61:        banner "$SSL_TRUSTSTORE_DIR does not exist; no certificates to import"
63:    elif [ ! "$(ls -A $SSL_TRUSTSTORE_DIR)" ]; then
64:        banner "$SSL_TRUSTSTORE_DIR is empty; no certificates to import"
68:    for cert in $(find "$SSL_TRUSTSTORE_DIR" -type f); do
168:export SSL_TRUSTSTORE_DIR

HTTP_API.md
1274:    `500` - The `TRUSTSTORE_DIR` environment variable is not set, or there is an unexpected error. The body is an error message.

Could you update that line in HTTP_API.md while we're at it?