Deployment fails with missing "cmake"

Hi everyone,

Trying to deploy an app to shinyapps.io, but installation of one of the required libraries (symengine, https://github.com/symengine/symengine.R) fails with the following trace:

[2021-01-08T11:51:37.153871591+0000] Building R package: symengine (0.1.5)
/mnt/packages/build /mnt
Warning in untar2(tarfile, files, list, exdir, restore_times) :
  skipping pax global extended headers
* installing to library ‘/opt/R/4.0.3/lib/R/library’
* installing *source* package ‘symengine’ ...
** using staged installation
====== configuring symengine R package ======
== current working directory
/tmp/RtmpZA1F1t/R.INSTALL2a23e9ffe8/symengine-symengine.R-7d482ea
==
== cmake:
cmake is not in PATH, try looking for /Applications/CMake.app/Contents/bin/cmake
cmake can not be found
ERROR: configuration failed for package ‘symengine’
* removing ‘/opt/R/4.0.3/lib/R/library/symengine’
GET /v1/tasks/?filter=account_id:222738&filter=parent_id:850927873&count=100&offset=0 810ms
################################# End Task Log ################################# 
----- Deployment error -----
Error: Unhandled Exception: Child Task 850927873 failed: Error building image: Error building symengine (0.1.5). Build exited with non-zero status: 1
 
----- Error stack trace -----
Error: Unhandled Exception: Child Task 850927873 failed: Error building image: Error building symengine (0.1.5). Build exited with non-zero status: 1
Error during wrapup: length(max.lines) <= 1 is not TRUE
Error: no more error handlers available (recursive errors?); invoking 'abort' restart
Execution halted

Can someone suggest where to start troubleshooting? Works fine on my local Windows 10 64-bit R 4.0.3 instance, and deployment worked OK for an earlier version of the app which also included symengine (late last year).

Update: have tried both fresh GitHub version and CRAN binary (locally) - no luck.

Is this normal? (From the deploy log)

GET /v1/tasks/850972378/logs/ 910ms
[2021-01-08T13:42:41.373983078+0000] Execute script: packages/build/units.sh
+ set -e
+ apt-get update -qq
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://cran.rstudio.com/bin/linux/ubuntu xenial/ InRelease: The following signatures were invalid: KEYEXPIRED 1602869253  KEYEXPIRED 1602869253  KEYEXPIRED 1602869253
W: Failed to fetch http://cran.rstudio.com/bin/linux/ubuntu/xenial/InRelease  The following signatures were invalid: KEYEXPIRED 1602869253  KEYEXPIRED 1602869253  KEYEXPIRED 1602869253
W: Some index files failed to download. They have been ignored, or old ones used instead.
+ apt-get install -y libudunits2-dev
Reading package lists...
Building dependency tree...
Reading state information...
The following additional packages will be installed:
  libudunits2-0
The following NEW packages will be installed:
  libudunits2-0 libudunits2-dev
0 upgraded, 2 newly installed, 0 to remove and 110 not upgraded.

...

The libudunits2 comes from the units packages.

Perhaps you need to install the symengine required dependencies in ubuntu?